You and your collaborator(s) may have devised a setting, some characters and the plot of your story but this represents only the beginning of the creative process for collaborative writers. At some point, you are going to need to tackle the meat of your writing project, that is, the actual writing of chapters, scenes or whatever unit your novel is divided into.
The experience of collaborative writing has shown me that the entire novel writing process can be broken into five activities: ideas, plotting, writing, editing and the final edit. These exist both at the level of the whole novel and of the chapter/scene.

Ideas
There is no story unless you have some ideas. At the whole novel level, this means coming up with the concept of your novel (e.g. a comic fantasy in which Satan loses his job and is relocated to Earth) and the broad strokes of what might happen (e.g. Satan decides to accumulate an army of followers by forming a band and becoming a rock god). At the chapter level, it’s the things that might happen in that chapter (e.g. Satan blows all his cash on a kick-ass guitar and massive amp).
Coming up with ideas together is one of the brilliant strengths of collaboration. Ideas often come through conversation and bouncing ideas off one another. Writers love to fiddle with stories, twisting concepts round and putting a new spin on things. At the ideas level, collaboration is definitely an area where two (or three or four) heads are better than one.

Plotting
Ideas do not themselves constitute a story. They are beads of story, waiting to be strung together. Deciding how the ideas you’ve come up with interact with each other and what order they occur in is a matter of plotting. Whether your plotting is simply the jotting down of ideas in order or the writing of a complete synopsis, once you have plotted your novel or chapter, you should be able to follow the events of the story by following the notes you have made.

Writing
I’m not sure this one needs explaining. Writing is the putting of words onto a page or screen and results in a written story. No, it didn’t need explaining, did it?
The difficult aspect of collaborative writing is that writing must almost always be a solitary business. In fact, there are few activities that are as damnably anti-social as writing. And this applies to even collaborative writing.
Most collaborators don’t actually spend their days sat side by side, typing at the same keyboard and dipping their biscuits in one another’s tea. Collaborators tend to work at some distance, even in different time zones. If you’re going to try to write together then you need to be clever about it.

Editing
Unless you are a literary genius, no one writes the perfect chapter or sentence on the first attempt. At some point, every writer has to go over what they’ve written and correct mistakes and make improvements where they can be made. Some writers edit as they go. Master short story writer, JL Borges, would spend weeks agonising over the construction of individual sentences (it shows). Other writers write second, third, fourth and fifth drafts of their novels, rewriting and improving with each pass. However you do it, you need to edit your work.
The question for collaborators is whether they are going to edit their own words, each others words or not distinguish between the two. Editing can be brutal and the collaborators must be able to trust one another with their words.
The collaborative co-writers of the BBC comedy Blackadder, Richard Curtis and Ben Elton, set themselves one brutal but effective rule for editing: should one writer put a line of editorial red pen through any one of the other’s jokes, that joke would be gone for good. No arguments, no compromise.

Final edit
The whole is greater than the sum of its parts. The individual components of something can be individually perfect but it is possible that they just don’t work together. Once your novel is complete, you may have a dozen gleaming and beautiful chapters but that doesn’t mean you have a beautiful novel. At some point, someone needs to take an overview of the whole project and make the finished product work.
Editing can be done collaboratively. As editing is the changing, addition and, frequently, the deletion of text, it can be done by one or more parties at once. Assuming they have the same aims in mind, two or more collaborators can work through the novel and, like the sculptor chiselling away at anything isn’t an elephant, edit away anything that is not their desired story.
Sadly, reality probably isn’t that simple and your final editing processes will need to be more structured than this.

All of these roles (ideas, plotting, writing, editing, final edit) are important and deciding who does them and how they are done is crucially important for the collaborative team. As collaborators, you want to divide the roles in such a way that plays to everyone’s strengths but without there being an imbalance in individual involvement.
One way to tackle the roles is to give each of the five roles to a specific collaborator. Perhaps one person comes up with the ideas and plots the story. A second collaborator writers the novel, editing their own work. A third collaborator is overall editor and brings the entire novel together. This could be a very effective method, particularly if one person is full of creative ideas, the second has a winning way with words and the third has the organisational abilities and objectivity required of a good editor. Having said that, is the writer in that relationship doing all the donkeywork to bring another person’s ideas to fruition? Maybe. Maybe not.
The fairest working relationship might be one where each collaborator participates in each of the processes equally. We’ve written elsewhere about planning and plotting together. Is it possible to write and edit together as well?
It’s not as easy as it sounds. Modern collaborators do not, except in exceptional writing teams, sit side by side all day long, working together, composing prose a word at a time. The workload must be passed around among them.

The most obvious alternative then is to act as a tag team. One collaborator can plot, write and edit chapter one, the next can work on chapter two and so on. There are two obvious issues with this strategy. Firstly, each chapter is going to bear the hallmarks of its individual writer. The plot twists, writing style and indeed the quality of writing may be appreciably different to the writer of the subsequent chapter. Furthermore, an entire novel written in this fashion may be as disjointed as a game of Consequences.

One step towards breaking it down is to alternate roles. For example, if Hilary plots chapter one for Leslie to write then Leslie can plot chapter two for Hilary to write. Each could then edit the chapter that they plotted but did not write. This works but one of the advantages of collaboration, that two people can work twice as fast as one, is arguably lost.
This again can be remedied by each writer starting work on a different part of the novel. Hilary plots chapter three, while Leslies plots chapter six. Each then gives the plotting/synopsis for the other two write up. Once both have finished writing, the original plotter can go back and edit the chapter their partner wrote. If this is happening then it is clear that both partners need to know exactly what is happening in both chapters three and six. The initial planning must be quite precise.
I used chapters three and six for good reason. If collaborators are going to tackle their novel in a non-linear order then there is actually good sense in not starting with chapter one. Many writers need to find their way into their novel, just like an athlete taking it easy during the early stages of a marathon. Given the importance of the first chapters for setting tone, introducing character and enticing readers, it actually makes a lot of sense for the writer to do that limbering up in a middle chapter where they do not have to worry about introducing characters or showcasing some stunning opening scene. There is also sense in collaborators working on chapters that are some distance apart. If, for example, Hilary and Leslie start with chapters three and six, any interesting developments, tonal shifts or problems that are brought up by chapter three can possibly be dealt with through the writing of chapters four and five without necessitating a rewrite of chapter six.

The writing of a complete ten-chapter novel might look like this:

What Hilary does…
What Leslie does…
Plot chapter 3
Plot chapter 6
Write chapter 6
Write chapter 3
Edit chapter 3
Edit chapter 6
Plot chapter 7
Plot chapter 4
Write chapter 4
Write chapter 7
Edit chapter 7
Edit chapter 4
Plot chapter 5
Plot chapter 8
Write chapter 8
Write chapter 5
Edit chapter 5
Edit chapter 8
Plot chapter 9
Plot chapter 1
Write chapter 1
Write chapter 9
Edit chapter 9
Edit chapter 1
Plot chapter 2
Plot chapter 10
Write chapter 10
Write chapter 2
Edit chapter 2
Edit chapter 10

One of the great aspects of a model like this (I speak from experience) is that as both writers are doing the same thing at any one time, there is a sense of pace and momentum. Hilary and Leslie can set themselves mutually meaningful deadlines. Perhaps they agree that each will write a chapter once a fortnight or once a month. Furthermore, each has had a hand in every chapter and yet the actual time spent writing this novel could be half that it would take either of them individually.

But what about a greater number of collaborators? Well, with three collaborators, it breaks down even more easily. There are three specific roles in the day-to-day writing. So when Hilary and Leslie are joined by Gertrude to write a twelve-chapter novel, they work like this:

What Leslie does…
What Hilary does…
What Gertrude does…
Plot chapter 2
Plot chapter 6
Plot chapter 10
Write chapter 10
Write chapter 2
Write chapter 6
Edit chapter 6
Edit chapter 10
Edit chapter 2
Plot chapter 7
Plot chapter 11
Plot chapter 3
Write chapter 3
Write chapter 7
Write chapter 11
Edit chapter 11
Edit chapter 3
Edit chapter 7
Plot chapter 12
Plot chapter 4
Plot chapter 8
Write chapter 8
Write chapter 12
Write chapter 4
Edit chapter 4
Edit chapter 8
Edit chapter 12
Plot chapter 5
Plot chapter 9
Plot chapter 1
Write chapter 1
Write chapter 5
Write chapter 9
Edit chapter 9
Edit chapter 1
Edit chapter 5

How would this work with four collaborators or five or more? The greater the number, the harder it is for all individuals to say what happens in each stage. There can also be an element of too many cooks spoiling the broth but I am sure that with dedication and organisation, larger collaborative groups can be made to work.
You will also notice that above I switched the three-way collaboration to a twelve-chapter novel so that each collaborator has four chapters to write each and one isn’t left over at the end to make the tea and sweep the floors. Of course, the job of collaboration is never over. There’s the final edit to prepare for, proof-reading to do, preparations for the next stages on the road to publication but nonetheless there’s a niceness, neatness and fairness in ensuring that all people do all things equally.