^ good advice.
A lot of it is experience based and if you can import that experience you can go faster and be better. Other than that, dissecting existing softwoods is a great way to learn more. Basically you are turning a 2D sheet good into a 3D object, so you can get far by studying other examples, making some quick dirty protos with fabric from JoAnnes and a hot glue gun (easier than stitching) and even paper mock ups.