These days, I do most of my layout in InDesign, even for printed materials, but some printers require me to submit Illustrator data. Here are the steps I take when creating in InDesign until the last minute and then converting to Illustrator submission data at the end.
Note: This is risky because it involves outlining text and relinking images. This method is imperfect. Ideally, of course, everything should be created entirely in Illustrator from the beginning.