Before writing any lesson, set a goal for the course (e. g. level A2 of the European Reference Framework for Languages) and make a lesson plan that will gradually lead people there.
A lesson plan should indicate which vocabulary and grammar each lesson introduces, and the order of which these are introduced. This gives current and future contributors an overview of what has been taught already. Already covered grammar and vocabulary should be regularly revisited. Grammatical structure that hasn't been covered yet will more often than not just confuse the reader, but if handled correctly it can be used as a tool to get readers used to certain modes of speech.
Avoid choosing which phrases to teach first, and then seeing what grammar they require. Once you have set up grammar points, you should pick sentences in their scope which the reader might find interesting and can begin to use in mock- or real conversations. What constitutes as "interesting" varies of course. At the outset, however, anything that readers can understand and use will encourage and motivate them. As most wikibooks are for self-study, a constant experience of success is a critical factor in the success of your book.
Keep the lesson contents manageable and their size regular. Again, centering around phrases will create some very short lessons, while complex phrases will require lots of background. Set each lesson to cover a small number of grammar points, introduce a set amount of new vocabulary and revisit particular grammar points.