This seems to be something most people struggle with - what to attach the screen back on with. Tesa Tape is a good option, otherwise in my first replacement I actually used a spare LCD Adhesive set I had lying around (refer to next step). My best idea was to use some Apple Watch Series 5 (44mm) adhesive, and cut it down.

One note is to not use the red heat-to-melt Apple Watch Adhesives (first photo), as this was my original plan, which unfortunately didn't work. I've found commonly the adhesives themselves don't actually say they're heat to melt, but you can tell from the colour.

Over the coming steps, I would highly recommend putting the adhesive on the main frame rather than the top cover, as it's much easier to align and get it in the right places. There are two nibs (top and right when looking down at the Pebble) that need to be adhesive free, as they clip into holes in the top cover for alignment.