Tips

The backrest should be adjustable and follow the shape of the spine.
It should also support the curve in the lower back.

Feet should rest flat on the floor comfortably.
If not, adjust the chair height or add a footrest.

Arm rests should be close the body and allow the shoulder to relax.

In a sitting position looking forward, the center of the computer screen is
what should be seen.

The back of the chair should come to the middle of the shoulder blades
in order to provide adequate support, above the shoulders is even better.

The seat of the chair should be long enough to put two or three finger
lengths between it and the knee.

If possible, the cushion should be memory foam.
Other cushions wear out quickly and make the chair uncomfortable.