Moving on to the Arm Rest Position; assuming you have arms on your chair, amazingly many don’t. Personally I would fall out of mine if it didn’t have arms.
Arm rests are designed for supporting your arms and elbows, not your body weight. With your arms on the arm rests your shoulders should be square, and relaxed, not hunched up to your ears. Arms close to your sides, not sticking out like cab doors. You won’t believe how much pressure this can take off your neck. If your chair doesn’t have adjustable arms, get them.
Now we are getting into some really complicated how-to.
First off to be truly ergonomic the arms of the chair need to be height, width and front to back adjustable. That’s not to say that less than optimal is not helpful.
Height is pretty simple, a lever or button located on the arm or under the arm cap will release the arm to slide up and down.
Width adjustment can be a knob or lever on the bottom of the arm that releases the arm to pull in and out, a sliding arm cap, or a pivoting arm cap.
Front to back adjustment is a sliding motion.