Set the machine to a straight stitch
Thread the machine correctly
Insert a fresh needle
Use matching thread in the top and bobbin
Test on scrap fabric first
Turn the top tension dial slightly higher to tighten
Turn the top tension dial slightly lower to loosen
Adjust the bobbin tension only if needed
Check the stitch balance on both sides of the fabric
Re-test after each small adjustment
Clean lint from the tension disks and bobbin area
Re-thread the machine after changing tension
Consult the machine manual for the correct tension range