I am in a similar situation and want to move from straight-line to free-motion stitching. Our guild had mentor night this month and machine quilting was one of the demos.
The lady said to first make sure that your setup is at the right hight. If your table is too high or the chair too low, you will have to shrug your shoulder UP.
She also said to anchor the quilt with all the straight stitches first. That way you can focus on the "between" areas and not have to worry about shifting.
She recommended to start on a community service quilt to practice. Not that the recipient doesn't want a nice quilt, but they are going to be less critical if it isn't perfect.
Lastly, some of the ladies use preprinted panels or the fabric on the back for the free motion work. The print provides a pattern you can follow and you don't have to worry about seams.