Set up the desk first
Most slouching starts with the equipment. A screen that sits low pulls your head down. A screen that's too far away pulls your whole upper body toward it. A chair you perch on the front edge of gives your back nothing to rest against. Fix those three and good posture stops being a fight.
The quickest test: sit back fully in your chair, look straight ahead, and check where your eyes land. If they land below the top of the screen's content area, you're fine. If you're looking down at the middle of a laptop screen, raise it. Books work as well as a stand.
Then sit in a neutral posture
OSHA describes neutral posture as one where your joints are naturally aligned, which reduces strain on muscles, tendons and the skeleton. At a desk that means your head is level and in line with your torso, your shoulders are relaxed, your elbows stay close to your body and your back is supported, whether you sit upright or recline slightly.
You don't need to sit bolt upright all day. A slight recline with the backrest taking your weight is a neutral posture too. The thing to avoid is the slump: hips sliding forward, back rounding, head drifting toward the screen.
Move more than you think you need to
Even a perfect posture becomes a problem if you hold it for hours. OSHA's guidance is to change position frequently: small chair adjustments, stretches, standing up and walking around for a few minutes, and doing some tasks standing, like phone calls.
Keep it up past the first twenty minutes
Everyone sits well for the first few minutes after reading a guide like this one. Then you start concentrating and your body drifts back to its habit. You don't notice, because noticing your posture is exactly what you stop doing when you focus.
That's the gap a webcam check fills. PostureCheck learns your upright posture in three seconds, then watches for your head dropping, moving toward the screen, your shoulders tilting or you sinking in the chair. When you slouch for longer than your alert delay, you hear a soft chime. It runs in your browser and nothing is recorded. You can also test yourself with the forward head posture test.