I'll go with what eyestar~* says.

Personally, and as I've told others, do not be too worried/concerned about writing each challenge in order. You can write whatever you want if the muse for any particular challenge hits you at the moment. You do not have to wait.

You can write all of them at any time, store them somewhere in your computer/wdc folder or book, and then simply post when you are ready to do so (in this forum) by chapter order.