You can always take classes and read books, but the best experience is by doing. In general I got better by slowing down and being methodical and allowing myself play time on the machine. Just by me trying things out without an actual garment in hand to finish is really a lot of fun. I went over and over (and over) how to do welt pockets.
Say I want to understand button holes. I'll do button holes for a few hours. Or welt pockets. That was all day. I totally feel comfortable with them now feel like I can do them in my sleep these days...lol. Hem and seam treatments? Same thing. Just allow yourself to play without the pressure of building a garment and its amazing what you do catch on to.
I also read this book
https://www.amazon.com/Pattern-Fitting-Confidence-Nancy-Zieman/dp/0896895742
Which is an amazing resource and, learning how to true a pattern, walk stitch lines, spot a problem before you get in deep is also amazing to do.
Doing a toile is another area were some don't like to go but I feel is super important. A paper pattern is a base to start from, the toile is what gets fitted to your body to perfection. It also becomes your pattern when you're all done fitting. Don't feel bad or awful if you're not doing a garment a day. I take weeks (months...lol )
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Evelyn wood is good start I think. https://www.youtube.com/watch?v=EDvqdZwAuTw
before I cut into expensive fabric, I make a muslin. I've got fabric thats 70.00/yard so, for that a muslin is key.
If you use a pattern by ANYONE, you have to know how much ease is put into the pattern. You also have to know how to change the pattern to fit your body if you don't have the "perfect pattern sized body"
https://www.amazon.com/gp/product/0896895742
this is such a common sense book, I was like a light went off and angels starting singing. some of the things I struggled with fitting, I got when I really read this book. There are some videos online as well, (bless this woman though she is no longer with us!)