The limit screws are small Phillips or flathead screws on the derailleur body, usually labelled H (high gear = smallest sprocket) and L (low gear = largest sprocket). They're typically at the back of the derailleur. Don't confuse them with the B-tension screw, which is usually at the top of the derailleur where it meets the frame.

Shift to the smallest sprocket (highest gear). The derailleur's upper jockey wheel should align directly below the smallest sprocket — view from behind the bike. If the chain wants to fall off the outside of the cassette, tighten the H screw (clockwise). If the derailleur can't reach the small sprocket, loosen it. The chain should run in a straight line from chainring to small sprocket with no sideways pressure.

Shift to the largest sprocket (lowest gear). The jockey wheel should align directly below the largest sprocket. If the chain threatens to fall off the inside of the cassette into the spokes, tighten the L screw. If the derailleur can't reach the large sprocket, loosen it. The chain should just clear the cassette carrier without rubbing.

Shift to the smallest sprocket. Loosen the cable pinch bolt, pull the cable taut by hand (firm but not yanked), and tighten the bolt. This gives you a neutral starting point. Now wind the barrel adjuster at the derailleur (or rear of the shifter) to the middle of its range — roughly 3–4 full turns out from fully tightened.

While pedalling, click through all gears from small to large. The chain should move to each new sprocket promptly after one click. If it hesitates or refuses to shift to a larger sprocket, there's not enough cable tension — turn the barrel adjuster counter-clockwise (out) by half a turn. If it over-shifts and jumps two sprockets, there's too much tension — turn clockwise (in) by half a turn.

Shift from the large sprocket back toward the small sprocket. If the chain is slow to shift down (toward smaller sprockets), add a tiny bit more cable tension — counter-clockwise on the barrel adjuster. If it double-shifts down, reduce tension slightly. Perfect indexing means one click = one sprocket in each direction with no hesitation or noise.

In each gear, the chain should run silently. A ticking or rubbing sound means the chain is being pushed against the next sprocket — the derailleur is sitting slightly off-centre for that gear position. If it ticks when in a middle gear: add a quarter-turn of barrel adjuster tension if the tick is on the upshift side, remove a quarter-turn if it's on the downshift side. This is the fine-tuning phase — small moves, test after each one.

Shift to the largest sprocket (most important position for B-tension). The gap between the top jockey wheel tooth and the bottom of the large sprocket teeth should be 5–6mm for most Shimano derailleurs, or follow the manufacturer spec for your specific derailleur. Turn the B-screw clockwise to increase the gap (moves the jockey wheel away), counter-clockwise to decrease it. Too large a gap causes slow, imprecise shifts. Too small a gap causes chain drag and noise on the large sprocket.