Once the stem bolts are tight, the top cap does nothing. You could take it off.

Loosen the stem before touching the cap, every time

The whole page in one instruction, and Park Tool puts a dedicated reminder in their own procedure because it is the step that gets skipped. The reason is mechanical: the cap compresses the bearings by moving the stack and the fork relative to each other, and a clamped stem prevents that movement entirely. One account describes exactly what happens if you get it backwards, which is that you are just putting tension on the star nut rather than on the bearings. Another names it as a mistake in the same terms, that the stem must be loose or the top cap will yank against itself. Nothing warns you. The bolt turns, it gets tight, and the bearing adjustment is unchanged.

The cap is a preload bolt, not a clamp, and 2 to 5 Nm is the range

The second half of the same misunderstanding. Because the cap is the most prominent bolt on the bike, it gets treated as the one holding everything together, and torqued accordingly. Guidance is explicit that the top cap is a preload bolt rather than a clamp, that over-torquing strips star nuts and crushes carbon expander plugs, and that typical torque is only two to five newton meters, with cranking it to ten listed as a named mistake. The proof that it is not structural is that once the stem bolts are tight, the cap and bolt no longer do anything and could be removed entirely. It is a tool that happens to live on the bike.

The steerer must sit below the top of the stem, or nothing you do will work

A precondition rather than an adjustment, and the most satisfying mechanics here. Park Tool specifies the steerer should sit about three millimeters below the level of the stem, because the stem needs to press down on the spacers to adjust the bearings, and if it is level with the top of the stem another spacer is needed below the stem. The reason a proud steerer defeats everything is a closed loop: the cap bears on the steerer, the anchor it threads into is inside that same steerer, so you are compressing the steerer against itself and no force reaches the bearings. Another guide puts the requirement as at least three to five millimeters of steerer or spacer above the stem clamp, and states flatly that a flush stem will not preload.

Set preload by feel and stop early, using a quarter turn at a time

A precise method that beats guessing. With the stem loose, turn the top cap bolt slowly, a quarter turn at a time, while rocking the bike with the front brake locked to feel for play in the headset. The instant the play disappears, add another eighth of a turn and stop. That is proper preload. Going further does not make the front end more secure, it puts a load on the bearings that they carry for their whole life, and the symptom is steering that feels notchy or resists returning to center. Too little and the headset knocks, which hammers the bearing races. The window is narrow and the eighth-turn rule is how you find it without a measurement.

A carbon steerer needs an expander plug and a spacer above the stem

Two carbon-specific requirements with real reasons behind them. On the anchor: patent literature explains that star nuts were acceptable when steerers were steel, but with aluminum and carbon the damage a star nut can cause when hammered into position is unacceptable and potentially dangerous, and once installed it cannot be removed without further damage and destructive deformation. So carbon takes an expander plug, which is set by expanding it inside the steerer before any preload is applied. On the stack: with an aluminum steerer it is fine for the stem to be proud, but with carbon the strong recommendation is a five millimeter spacer above the stem so that the stem is entirely clamped around the steerer and cannot crush the top edge. An unsupported carbon edge inside a clamp is exactly where a steerer cracks.

Grease the bolts and never the clamping surfaces

A distinction that is easy to get backwards when you have a grease gun in your hand. Park Tool says to lubricate the adjusting bolt and the stem bolts if they are dry, and then states the exception in capitals in their own text: do not lubricate inside the stem or on the steering column surface. Those are friction surfaces, and the stem holds position by clamping friction, so greasing them is actively removing the thing keeping the bars from rotating. And a second material note worth having: lithium grease can attack some elastomer seals over time, so use a marine or bicycle-specific grease rather than whatever is in the garage.

Two silent failures: the missing compression ring and the bottoming cap

Both produce the same symptom, which is a cap that turns and play that does not go away, and neither is obvious. The compression ring is the small wedge ring sitting above the upper bearing, and guidance is blunt that it is essential and that without it the fork will not preload at all. It is easy to leave out on a reassembly because it looks like a spacer. The other is subtler and comes from somebody correcting a published article: top caps with a large step down in the middle can bottom out on the top of the expanding bung or star nut before enough preload has been applied. In that case the bolt genuinely does stop turning, it feels correct, and the bearings were never compressed. If the cap tightens up but play remains, check both of those before assuming a worn headset.

Align the stem before torquing it, and torque it properly

The last two steps, and both have a specific method. For alignment, snug the stem bolts only until you can still turn the handlebars independently of the front wheel, then get the stem, the top tube and the wheel all in line, and only then torque fully. Trying to align a fully clamped stem means loosening it again, which risks disturbing the preload you just set. On torque, both directions are failures: an under-torqued stem will slip, which on a front end means the bars rotating under load, and an over-torqued one can crush a carbon steerer. Use a torque wrench and the figure printed on the stem rather than a feel, because the stem is the part actually holding the front of the bicycle together now that the cap is doing nothing.