Which wrist should you wear a watch on?

Traditionally the wrist of your non-dominant hand, so the left if you're right-handed. It's less likely to get knocked, and the crown on the right side doesn't dig into your hand. There's no rule, though: wear it where it's comfortable.

Should a watch sit above or below the wrist bone?

Just above it, on the forearm side of the bone on the outside of your wrist. That keeps it from sliding over the joint when you bend your hand.

Can you wear a watch with a bracelet?

Yes. Wearing them on opposite wrists avoids scratches, but stacking a watch with a bracelet on the same wrist is a common look; put softer bracelets next to the watch.

How to Wear a Watch: Which Wrist, Where, and With What

How to wear a watch

Wear it on your non-dominant wrist, just above the wrist bone, snug enough that it doesn't slide around. Everything else is style.

Which wrist

Most people wear a watch on the wrist of their non-dominant hand: the left for right-handers, the right for left-handers. Your dominant hand does more, so a watch there gets knocked more, and on most watches the crown sits on the right of the case, where it would press into the back of a left hand. Some watches, called destro or left-handed models, put the crown on the left for right-wrist wearers.

None of this is a rule. If you prefer your dominant wrist, wear it there.

Where on the wrist

Find the round bone on the outside of your wrist, the ulnar styloid. The watch should sit just above it, toward your elbow, so the case doesn't ride over the bone when you bend your hand back. With a dress watch you want it to slip under a shirt cuff; with a dive or sports watch it can sit a little higher.

How big a watch looks depends on its lug-to-lug, not its diameter. If the lugs reach past the edges of the top of your wrist, the watch overhangs and looks too big. The fit finder shows which watches fit your wrist.

How tight

Snug, with room to slide one finger under the strap. A watch that's too loose spins and slides down your hand; too tight and it leaves marks and gets uncomfortable as your wrist swells through the day. More detail in how tight should a watch be.

Matching a watch to an outfit

Suits and formal wear: a thin dress watch on a leather strap that disappears under a cuff. Match the strap to your shoes and belt if you like the old rule.
Business casual: almost anything that isn't a chunky diver or digital watch. A steel everyday watch on a bracelet works everywhere.
Casual and outdoors: field watches, divers and G-Shocks, on canvas, rubber or a NATO strap.
Metals: it's nice, not essential, to keep the case metal in step with your other jewellery and belt buckle.

Questions

Which wrist should you wear a watch on?: Traditionally the wrist of your non-dominant hand, so the left if you're right-handed. It's less likely to get knocked, and the crown on the right side doesn't dig into your hand. There's no rule, though: wear it where it's comfortable.
Should a watch sit above or below the wrist bone?: Just above it, on the forearm side of the bone on the outside of your wrist. That keeps it from sliding over the joint when you bend your hand.
Can you wear a watch with a bracelet?: Yes. Wearing them on opposite wrists avoids scratches, but stacking a watch with a bracelet on the same wrist is a common look; put softer bracelets next to the watch.