the best way i have found to practice is first just sandwich some fabric and doodle. Try to find the balance between the speed of your machine and your hand movement. Use a medium speed.
Then practice the following = loops or e's, then a meandering, then a wave with curves and points and then long wiggly lines. These are the four basic shapes you will find in fmq. Make these motifs about the size of a 25 cent circle or even a 50. circle. Most practice too small so you have to make yourself do larger motifs. Don't try and make "designs" at first. Practice these 4 until you get good curves and lines. I find also that most students when doing the wave and point, make them look like sharks teeth. When you are doing the curves and points work in one ful sweep up to the point and pause just a fraction of time before going on. This will give you a good sharp point otherwise the stitches tend to jump from one side to the other.
Try to imagine the movements in old fashioned penmanship exercises with ovals, circles, up and down, gack and forth. The most important part is finding the balance between your machine speed and the speed of your hands to get the quality and size of the stitch you want. Then practice, practice and practice........One doesn't learn good fmq overnight or in one 30 minute practice session