I’m going to show you a simple way that I sometimes make a clothing sketch of a clothing piece that already exists. If I have to dream something up, I have to work harder and do all the work myself but in a case like the Ramona Sweater I’ve been working on, it’s a piece of clothing that has photos and I can copy those.
I start out by printing out a photo of the sweater I need to make sketches of and then I use a sharpie marker and outline the shape I want to copy. Here it’s the sweater itself and a few little details like neck and hands just to give some idea of body placement.
Next I go to a window (or if you have a lightbox, use that) so you can see the design through the paper and trace it. I trace as many sweaters as I need for my ideas to flourish.
Then I color it in. I’m not always super careful about coloring but I don’t feel I need to be in some cases. Here, I just need to get the overall picture in my head of what I want my sweater to look like. If I was doing a fair isle or something where detail mattered then I would most definitely work harder at coloring more carefully.
If you dislike sketching and doing the body drawing, there are places like My Body Model that will help you make a croquis of yourself. This can also be helpful if you want your sweater shape to be more like your body and not just some general body that you found online.