Positioning Yourself
Sit close to the desk and don't put things between you and the keyboard except a gel wrist rest. Your elbows need to be vertical under your shoulders. Don't automatically use the keyboard's rear feet. You only need them if your elbows are below desk height. If you rest your wrists on the table when you type, you should use a wrist rest.
Position and use the mouse as close to the keyboard as possible. Aim to have your elbow vertically under your shoulder and right by your side or on the armrest.