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.