My practice is usually on a quilt. I made a giant king-size quilt for the purpose of practicing feather motifs in different shapes and sizes - there are probably a hundred or 150 feathers on that quilt. The top is so busy, you can hardly see the quilting - but if you look at the back, you can tell that this is the most over-quilted scrap top ever made in history.
I also buy quilt tops - I don't like piecing very much and I can't get tops finished fast enough to get on with the next quilting session.
Christine Maraccini's book on machine quilting is one of the best ones out there - for each quilt top, she has three examples quilted for different purposes - one is intended for heavy use and lots of laundering, (she calls that one a "dragger," because it will be dragged around by a child everywhere s/he goes) one is intended to be a keepsake/heirloom and one is intended to be a show-stopper. The quilting gets progressively fancier and more difficult going from "dragger" to "show-stopper." She has a lot of beautiful motifs and shows you step by step how to go from one to the next.
One thing that is a big help is drawing the motifs with pencil and paper or on a Dry Erase board. Draw the motif over and over and over until you're sick of drawing it. Draw it until you no longer how to think about where you're going next - getting into and out of tight spots is automatic.
For some reason, this complete confidence with the design translates itself to drawing with a sewing machine and thread. When you sit down to quilt it, it seems to come out of the needle almost by itself. The tension will be gone from your shoulders and you will breathe without having to remind yourself to do so. So draw, draw, draw.
Then go to a practice sandwich and after doing some quilting on it, check the tensions by examining the stitches on both sides. Make any necessary adjustments and then go on to the quilt.
You can do it - don't be afraid of it and don't get frustrated with yourself. It takes time and practice for most people. Have fun!