You can see in the vid I made that I apply the sap to the hot wood. As the wood cools it pulls the hot and still liquid sap into itself and it is surprising how deep it gets pull in. I have heated a limb after scraping the wood and removed all evidence of heat treating and you can still see sap oozing out from the heat.
As to collecting the resin/sap, I just go around and pick it off trees where they have been injured. Not too hard to collect enough to do a dozen bows or more