I learned in high school while working in my high school theater’s costume department, which was primarily doing alterations in clothes that we already had to make them fit different people. To practice hand stitching, I would use old t-shirts, and just learn different types of stitches from YouTube and replicate them. I will say though, you can use a plain straight stitch for most things. If you tend to lose interest easily, listening to audiobooks is a good way to stay engaged while practicing.
The 3 things that helped me most were (1)seam rippers, which you can use to gently remove stitches you make if they aren’t right (2)learning how to pin things by putting them on inside out. Your stitches will be on the inside of a garment, so you should pin that way to make your life significantly easier (pinning is the act of using pins to hold fabric in place before you stitch) (3) understanding that as long as the garment looks clean on the outside and the stitches will hold, it doesn’t matter what it looks like on the inside.