An office chair is not a posture machine. Its job is simpler: make the work in front of you easier to see and reach, while giving you enough support to change position comfortably. Start with the chair before changing everything else, then revisit the setup after a normal work block.
Set the height for the task
Sit far enough back that you can use the backrest when you want to. Adjust the chair so your feet have a stable place to rest and your thighs are comfortably supported. Then check the desk: can you use the keyboard without lifting your shoulders or pressing your forearms hard against the edge? If the desk is fixed and too high or low, use the most useful compromise and make other changes around it.
Check seat depth and back support
Slide the seat pan so it supports most of your thighs without pressing uncomfortably behind your knees. Use the backrest as a place to return to, not as a shape you must hold all day. If your chair has adjustable lumbar support, place it where it feels supportive during ordinary work rather than trying to force a particular curve.
Let armrests help, not block you
Armrests can provide a brief place for relaxed forearms, but they should not prevent you from getting close enough to the desk. If they push your shoulders up, collide with the desk, or make you reach for the keyboard, lower, narrow, move, or temporarily avoid them if the chair allows. Your keyboard and mouse position matters just as much as the chair controls.
Test the whole arrangement
Open the work you normally do and spend a few minutes typing, mousing, reading, and taking a call. Notice the repeated urges: leaning toward the screen, reaching for the mouse, perching at the front of the seat, or tucking one leg because your feet have nowhere comfortable to rest. Change one variable at a time so you can tell what helped.
Keep changing position available
Even a well-adjusted chair is not meant to keep you still. Shift, stand at a natural transition, and take short walks between focused blocks. The sitting posture guide explains why variety is more useful than chasing one rigid pose, while the desk-breaks plan offers simple reset ideas.
If pain, numbness, weakness, dizziness, or symptoms after an injury are part of the issue, seek individualized guidance from a qualified healthcare professional. This is general setup education, not medical advice.