Use tweezers to peel up any adhesive left behind on the lower case.
Clean all traces of adhesive (and broken glass, if any) thoroughly from the case before re-sealing your watch. Scour the case using the tip of an opening tool or spudger wrapped in a lint-free cloth or coffee filter, along with a little isopropyl alcohol (90% concentration or greater).
Don't rush this step. Even a small amount of residue can prevent your display from bonding to the case, causing it to come loose over time.
If you're re-using your existing screen, this is a good time to remove the old adhesive from there as well. (If you're installing a new screen, ignore this step.)
Use a plastic opening tool to scrape off the old adhesive around the edge of the screen. If possible, peel off larger pieces using tweezers.
Thoroughly clean the area under the adhesive with isopropyl alcohol.
If you're using a replacement battery that came with pre-installed adhesive, peel off the protective tab now, exposing the adhesive.
If needed, try re-using the existing adhesive to secure the battery. Alternatively, try a small piece of double-sided tape such as Tesa 61395, or a dab of liquid adhesive such as E6000.
Rotate the battery down into its slot, and press it into position.
Press the battery down against the system board to re-adhere it in place.
Now is the best time to test powering on your watch and test the force touch function—if it doesn't turn on, double check all three connectors and try again.