This is a bit tricky as it depends on what you removed, often times the ForceTouch sensor gets damaged so you need to replace it and that only has adhesive on one side. Then you have the second dual sided adhesive die-cut which you first adhere to the ForceTouch and then peel off the blue protective layer to then mount the crystal display.
Now before you even start this, the surfaces need to be spotless and have nothing getting in the way. I’ve also found some cleaners are not leaving the surface clean enough. As an example I use Goo Gone which is a citrus oil to remove the adhesive (which it does quite well) but the film it leaves a surface that the adhesive won’t adhere to properly. So I do a second clean with 90% reagent grade isopropyl alcohol going over the surface twice with a clean wipe.
Then we get into the heat and pressure, the adhesive doesn’t like cold surfaces as such I use a heat pad to warm the case and crystal display just before I start applying the parts (it doesn’t take much about 130F/54.4C). After the crystal display is mounted, I continue to apply heat as well as place a heavy brick to top applying even pressure across the crystal and covered of course so it doesn’t scratch the crystal. I leave the heat on for about an hour and then let the watch set for the night with the weight.