What do the controls on my Garmin watch do?

Logging a set on the watch is two steps, each on its own screen, and the same Up/Down buttons (or swipe up/down, depending on watch model) drive both — they just adjust a different number depending on which screen you're on.

Step by step

1. Weight screen — starts on the prescribed weight. Up/Down or swipe adjusts it; presses close together ramp the step size (0.5 → 1 → 2.5 → 5, in whichever unit you display) so dialing in a big change doesn't take forever, while a single tap still gives you fine control. Handy if you're on a different machine than planned. START moves to the reps screen; BACK returns to the set overview.
2. Reps screen — starts on the target rep count, not blank; Up/Down or swipe adjusts it up or down from there to match what you actually did. START logs the set (prompting for RIR first if that's turned on) and advances to the rest timer, or the workout summary after the last set. BACK returns to the weight screen for the same set, if you need to fix the weight first.

Good to know

Anything you log on the watch syncs back to the phone/desktop app the same way a session logged on the phone syncs out to the watch — they're equal peers, so it doesn't matter which device you finish a workout on.

Related: "What is RIR (reps in reserve)?", "Account & watch".