Hi John,
Carefully remove the flap of bark, without damaging any viable tissues. Watch out for peel, when doing this, have some tools handy. If the bark edges left on the tree are realy rough, which in those type of wounds they typically are: use a sharp chisel, disinfected with rubbing alcohol, to edge the wound. That means carefully go around the edges of the wound, surgically removing the rough, or peeled back bark edges, to healthy tissue. Remove as little tissue as possible to achieve this, while making sure the tissue edges left are healthy and viable. If you can envision the bark layer as a loaf of bread, cut the bark layer straight, like a slice of bread, without damaging the
wood if possible. Don't use a hammer, use a knife and sharp chisel.
This will eliminate bug hiding places, so they can't leave their high nitrogen frass to speed up rot. It also eliminates places moisture can linger, to minimize rot, while letting the sunlight better disinfect the wound. It also leaves clean surgical cuts on the canbium layer, which often speeds up the generation of bark tissues to more quickly close the wound, and help compartmentalize the potential for pathogenic rots.
Hope that helps