There are vast varieties of topics on designs, but the best information you can ever lay your hands on is in the 2nd edition of Origami Designs Secrets by Robert Lang. it's expensive though, it's worth the price.

For the basics you want to get acquainted with, I get to design models by taking a close look at the placement of each appendage on the subject matter. After which I'll apply a flap location to each appendage and as well work-out their respective CPs using 22.5deg technique or box pleating from there. The best way to doing this is to experiment with oneself and note that designs are a largely trial-and-error phenomenon.