One big mistake I made when I started free motion quilting was: I forgot to put the darning foot down! The darning foot does not sit down on the fabric as other feet do, and if you aren't paying attention, you think everything is set, and then you will get huge loops underneath (ripped out the same quilting THREE times before I found out what I had forgot to do).
Also ... it does take time and practice to get stitches the same size ... heck! You can't even do that with a speed regulator on your quilting frame and sewing machine (have no experience with mid-arm or long arm quilting) ...
You are not going to do anything but get better from here on out, and the advice about "writing' your name and alphabet (practicing penmanship as it were) is the best practice you can do!
Our local quilt shop has classes for learning how to use a long arm, and of course, we work on samples for that class ... it is just about using the thing, not set up, etc ... anyway! They make a quilt of muslin, batting, and muslin ... when the little quilt is completely covered by students or whomever, they just bind it up and send it to our local humane society ...
the potholders were also a good idea, because who wants to practice and then just throw the thing away? I don't! LOL and, most of us don't want to spend time doing anything that will go from machine to trash ... so, I thought I would share those thoughts, also.
Really enjoyed the comments on the Flynn quilting System, also. I have talked myself in and out of the concept, I can't tell you how many times ... I feel a change of mind coming <g> LOL again ....