Pinups are drawings or paintings that focus on the human form with minimal backgrounds. The following example is focused on the woman and has just enough props and background to tell the viewer a story, but not over power the figure.
1) Gesture Drawing
Starting with a picture or idea create a gesture drawing that is loose and fluid, but captures the action or energy of the pose.
Using your gesture drawing, create a proportionally correct stick figure and block them in using basic forms, cones, cylinders, and spheres. Follow the link to human proportions
3) If the figure is clothed in loose or heavy clothing you can usually dress them at this point without adding muscles. If the figure is wearing tight or thin clothes then add musculature before adding clothing.
4) Lastly add clothing and other details to finish out the figure. Clothing is made from cylinders and cones just like a person so draw them accordingly. Clothing folds where that material is compressed creating an accordion pattern, usually on the inside of a joint (back of the knee) and is stretched on the outside of a joint (front of the knee).