Learning pattern drafting is actually fairly easy.
I am not meaning this to imply that it is easy in any way to create a good professional sewing pattern. The practice required for professional level skill takes, well, professional-level work. You have to design to a wide range of body types, making a sewing pattern that will be flattering on the largest range of figures possible.
I don’t know how to do that.
Professional design ability also means that you know how to create a very wide range of design elements suitable for not only current fashion trends, but in encouraging other fashion looks.
I don’t know how to do that, either. I am not by any means a professional clothing designer.
What I can do is take a set of measurements, make a sloper, and then design something relatively basic from that. The results are so much nicer than a commercial pattern (custom usually is) that fancy design elements are almost unnecessary. Never underestimate the beauty of well-fitting clothes.
The thing is, I’ve noticed that people who are used to commercial patterns will sometime freak out at dart width and placement on something they’ve created for themselves, especially women who wear quite a large cup size, or have a bust measurement a great deal larger than the waist measurement. If you’re used to the two inch dart on a commercial pattern designed for the “average” B cup young woman with perky breasts, that seven inch dart for the F cup designed for the woman of a certain age who hasn’t resorted to surgery looks downright freakish when drawn flat on the fabric. It’ll make you scared to cut the pattern out.
But after you sew it… Ahhh, what a thing of beauty a custom designed pattern is. Narrow shoulders? No problem. Ever had a armscye hit exactly at the shoulder joint? It really improves the look of the garment. A bust dart that actually shapes the fabric to your body and ends where your nipple actually is? It makes a much smoother line. A garment that has the waist where your waist actually exists, and flares out appropriately to the real wide point of your hips?
You can’t get that stuff in ready to wear, and even have to do a lot of pattern redesign to get it in a commercial pattern.
The book from which I learned how to do this (Patternless Fashions) is unfortunately out of print. Part of this is because the styles in it are dated and the instructions really aren’t as clear as they could be. But since the point of being able to draft one’s own patterns is that you can add any fashionable design details you want to, the fact that the drawings in the book look a bit 1960 doesn’t matter but so much.
If you like to sew and you like to make your own clothes, I can’t encourage you enough to learn to draw your own patterns. The fit is amazing.