You might already have Hours on your Apple Watch. When you set up your Apple Watch, you had the option to either Install All or Install Later. If you picked Install All and you had Hours on your phone, Hours should already be synced with your watch. Look for the Hours icon on your watch:

Also, any future updates to Hours on your iPhone will automatically sync to your Apple Watch.

If you do not yet have Hours on your Apple Watch, make sure you have updated to the latest version of Hours, then:
  1. Open the Apple Watch app on your iPhone;
  2. Tap on the My Watch tab at the bottom of the screen;
  3. Scroll to find Hours, and then tap it; and
  4. Turn on Show App on Apple Watch
For details on using the Apple Watch, check out Apple's Apple Watch User Guide.