How do you get into drafting your own patterns?? Sewing Machines Parts and Sales in Los Angeles

Semi-rant here. I started making a Jedi robe last week and pretty much gave up when the sleeve heads ended up about 3 inches down my biceps. I do pretty well at being a self-taught seamstress and most projects come out ok, but I’m tired of this roulette game where I never know if a pattern is going to fit my (apparently weirdly-shaped) body.

Seems like most people who post on here self-draft their own things, and I want to know how to get into it. Bought a book from amazon but am at a bit of a loss. Looked into classes at Joann and my local family-owned fabric store, but neither offer classes in it that I’ve found.

Can anyone recommend an online tutorial for me, or another book? I’m willing to pay for this. I just want to stop tearing my hair out over patterns that don’t fit and learn to make my own.