This may be off the subject of the loose thread on the backing....seems that is covered. Some hints I learned from and still use are to make a small sandwich using the scraps from the quilt you are preparing to quilt....test your stitching on this small practice piece. It is the very same weight and color of your quilt. You can see how the thread and stitches look on the actual fabrics. I learned this tip from one of by machine quilting books...don't remember which author....she recommends this and she uses this sample piece to "warm up" on before quilting on the actual quilt each time she sits down to quilt on the quilt--she quilts large quilts.
As mentioned before....practice is the most important. Learn to relax and take a breath. Find the machine speed that works for you. It is usually a medium speed with medium movement of the the quilt. I think first time machine quilters think they have to go really fast and they get really tense. Also, a good way to learn to free motion and get control of the machine is to get a child's panel and sandwich it for a quilt then free motion around designs in the panel. You can stipple and do different background stitching....a great practice source. You can then donate your quilt to one of the children's charity quilt groups.