There are 2 parts to tensioning your machine. I spent a weekend tinkering with my singer 301 to get the tension right, and play with the attachments that I have.
Start by tensioning for a straight stitch with a double layer of fabric with the goal of having the upper tension at the middle mark by trialing different positions on the bobbin case screw. If you are cranking the upper tension all the way to the top, the bobbin case tension is too loose. Tighten that screw and sew another seam. Keep doing that with the feed dogs up until you have a balanced straight seam.
Then drop the feed dogs, put on the darning foot or FMQ foot and sew sample straight seams with your quilt sandwiches adjusting the upper tension only until you get a balanced seam again. Once you get that, then practice moving the sandwich around to get nice looking stitches by varying the speed of the machine and the speed of your hands. If you have a machine that can regulate your speed try different settings along the medium to fast speed.
One other thing that really helped me was to watch some of the you tube videos on FMQ, Leah Day and Patsy Thompson were the ones I looked for, and eventually I bought a bunch of DVD by patsy Thompson to practice with. Those, some chocolat and a glass of wine ( along with a few choice swear words) really helped.
I use my Bernina 430 for most of my FMQ since I can vary the speed setting. It's not perfect by any means, but I do end up with a useable quilt
:-)