This is an easy way to set your machine thread tension. (Bobbin tension is
rarely needed, and then should only be attempted by those with experience,
or by a sewing machine shop. Some manufacturers will explain how to set
bobbin tension in their operation manuals, while some only recommend a shop
to set it. If in doubt, it is best to allow your repair shop to look at it
for you.)

1. Set machine with one color thread for the bobbin and a different color
thread for the top feed.
2. Sew a short seam using a scrap piece of cloth, 2 layers thick.
3. Look at the seam. If you can see the bottom thread coming through the top
of the fabric, turn the tension down.
4. Likewise, if you can see the top thread from the bottom, turn the tension
up.
5. Repeat the above until the top and bottom are even.