How do I set overlocker tension?

Change one tension at a time and use a different colour in each position. With four matching threads you cannot tell which one is wrong, and this is why overlocker tension has its reputation.

Start from the defaults. Set every dial to the value in your manual, thread with four different colours, and sew a sample on a doubled scrap of the actual fabric. Then read the sample.

What a correct stitch looks like. The two looper threads should meet exactly at the edge of the fabric and lock there. The needle threads should sit flat in the fabric, forming small even loops on the underside without pulling the fabric in.

Reading the faults.

Upper looper thread pulled to the underside — upper looper too tight, or lower looper too loose.

Lower looper thread showing on top — lower looper too tight, or upper looper too loose.

Needle thread loops loose underneath — needle tension too loose.

Fabric puckering along the seam — needle tension too tight, or differential feed set wrong.

Adjust in small steps — a half or one number — and re-sew the sample each time. Note the settings that work for each fabric type; overlocker tension is fabric-dependent in a way sewing machine tension is not.

Before touching any dial, check the threading. An unseated thread mimics every tension fault there is. Re-thread in the order the manual gives, with the presser foot raised so the discs are open, and check each thread has actually gone between its discs.

And check the needle is the correct type, new, and pushed fully up.