Hopefully, this will help. Tension is a tug of war between the top and bobbin threads. When the tension is correct, the two threads will meet in the middle of the fabric. If the top thread is showing under the fabric, increase the top tension so it will pull the bobbin thread up higher. If the bobbin thread is showing on top of the fabric, decrease the top thread tension so the bobbin thread will pull it down into the fabric. However, the above will work only if the bobbin tension is set appropriately. For the most part, tension should be governed by the top tension and the bobbin tension should be changed ONLY if top tension does not take care of the problem.NOTE: Remember to ALWAYS RAISE the presser foot when threading the top thread!!!
Understanding Thread Tension Threads A lot of people believe that tension is a "set it and leave it" option. It is not. Tension needs to be adjusted frequently, depending on the fabric, thread, needle being used. So, a tension that works for one project may need to be adjusted for the next project. It should be tested on scrap fabric made of the same fabric as the project. Once the tension is correct on the scrap, then it is ready to go on the project.