1. First be sure the type of thread is the correct one. As this machine (KS-810 or KS-820) is intended for medium-heavy and heavy materials such as canvas and leather. So, recommended thread Nylon Bonded #69.
2. Correct Threading.
3. Tension. Upper Tension and Bobbin Tension needs to be balanced. Start from low tension in both.
Where to adjust bobbin tension: See the bigger screw on the bobbin basket side.
With the Presser Foot down, pull the upper thread before entering the needle eye, thread needs to come out when you pull otherwise is too tight.
Bobbin Thread same thing when you pull the thread needs to come out with ease. Start from low tension balanced. When bobbing is too tight pulls down the upper thread creating a bird's nest.