First step in designing anything is an idea. I've tried to assemble a decent collection of things to help you achieve that. Some are things I've used myself, others are just ones that I've found and bookmarked in case anybody else uses them.
Free Fashion Croquis - I haven't used anything from here as I tend to need body templates to be closer to my own size, but maybe somebody else can get some use out of some of them.
Realistically Proportioned Female Croquis - I really like these. The sizes work a lot better for me, though I have to admit I would prefer a more neutral pose (which can honestly be said of most of the croquis I've found).
Duct Tape Form - Alley Cat Scratch tutorial. I loosely followed this one for my first duct tape form, which came out quite nicely and help up for several years.
Duct Tape Form 2 - Threadbangers tutorial. I haven't used this one, but I see it recommended enough that I figured I should include it.