I started out thread painting in the early 80's on a machine where you didn't use a foot when doing free motion thread painting, then I did a few trial quilt sandwiches and put it away for 20 years. When I started up again in the 2000's I just put together some practice sandwiches and started doodling. I didn't worry about stitch length or even what I was quilting, just played. Sometimes it just looked like a kid's scribbling, but even that helped get the control down. Then when Craftsy started up I began taking classes.
I say just jump in and play. It's only fabric, and fabric can be replaced. The more you play, the better you will get. I spend a fair amount of time FMQ doodling, using pieces of cheap craft store felt for my sandwiches. I find it very relaxing. I also give myself a break if I'm having a day when my stitches aren't all the same size, if they're sturdy and the quilt will stay together, then I'm good with that even if some are a little longer than others. Same with wobbles. "Mistakes" become an opportunity to either learn how to fix or to develop a new design. What I'm getting at here is: don't be too critical of your efforts, just enjoy the process. It's about having fun.
Rob