Focus the reticle first (diopter)
Before anything else, set the ocular/diopter so the reticle is razor sharp. Point the scope at a blank bright surface (sky), glance through for only a second or two, and adjust the eyepiece until the reticle is crisp on first look. Don't stare — your eye will compensate and fool you. A blurry reticle means your eye is straining and your aiming wanders.
Parallax — the silent group-killer
Parallax is when the reticle and target aren't on the same focal plane. The tell: if you move your head slightly and the reticle appears to slide across the target, you have parallax error — and your point of impact shifts with any tiny head movement.
Fix it with the side-focus / parallax knob (or adjustable objective). Adjust until, when you move your head behind the scope, the reticle stays locked on the target with no apparent movement. At 100yd, set parallax to 100. Do this for each distance.
Consistent eye position
Maintain proper eye relief (distance from eye to ocular) so you see a full, edge-to-edge image with no dark crescent ('scope shadow'). A crescent of shadow means your eye is off-axis and you're aiming with error. Consistent cheek weld gives consistent eye relief automatically.
Aim at the same precise point every shot. 'Center of the dot' must mean the same thing every time. For group shooting, hold dead center on one consistent aim point rather than chasing the last shot.