Full .pdf of the finished result is here: http://www.summerblock.com/Ikea_Baby_Book.pdf
Step 1: Gather necessary software, etc.
--3D modelling software; I used Google Sketchup Pro and 3dsmax
--A handful of Ikea instruction manuals in .pdf, available here: http://www.ikea.com/ms/en_US/customer_service/assembly_instructions.html. Pick ones with similar layouts to the manual you want to make.
--Fonts; I had good success with Twentieth Century Modern throughout (bold-faced for the title page). Oddly enough, the Ikea Sans font that comes in the online manuals doesn't look quite right to me. I was using an actual manual that came with my bookcase for comparison, and the capital A on the cover, in particular, was hard to match.
--A 3D model of the object you want to assemble. Either construct it yourself or Google "free 3d model _" and fill in the blank. I was only able to find a baby model in Sketchup format, but then I exported it as a .3ds and used 3dsmax.
--A liberal interpretation of copyright/trademark law. Hey, it's satire!
Step 2: Sketch out manual
After you have a rough idea, think in particular about what kind of images you'd like to use--which parts of the model from which angles, etc., and make a list of all of these.