I love free motion quilting, and have since my first quilt, when I was working on a $150 Brother machine with a 5 inch throat. I now have a Janome 6600, which has a 9" throat, which makes it easier to quilt larger quilts, but fmq is still the same. It takes practice. I do a lot of doodling before I even head to the sewing machine with a new design. I fill several sheets of scratch paper (back side of children's school papers.) with a new design until I can predict how the design goes and how i will work my way across the sheet filling it in. You build muscle memory and then transferring it to the fabric becomes easier. After I've doodled - a bunch- I go to scrap quilt sandwich made from pieces of old cotton curtains and batting scraps pieced together. I'll practice on these practice sandwiches until I get the tension, speed and stitch length right and can see if I'm skipping stitches. Then I work on the size of my motif. Do I want little flowers; or big ones? It's a lengthy process to get started actually quilting a quilt. I recommend practicing on your machine until you're confident, or just figure out how many quilts you can send out for the price of a new quilting machine. I look again and again at the HQ Sweet 16 sit down and think I really want one, then I consider how many quilts I could send out to have quilted for the $6000, and go back to my current method. I can comfortably quilt anything up to a twin sized quilt on my 6600 and I can manage up to a queen. It's only the really big ones. (kingsized ) That I send out.
Good luck with your quilting journey.