A torch or an alcohol lamp will do. Heat the end of the hot-melt and smear a small amount on the taper. Heat the ferrule of the broadhead enough to remelt the hot-melt when you put it on the taper, and rotate the head (use pliers!) a couple of times to spread the glue. You should have enough glue to see a little squeeze out when the head seats, but not so much that it keeps the ferrule from seating solidly. Press the tip of the head into something hard, careful to keep it perpendicular, blow on it to partially set the glue, and then immerse the head in cool water.
Spin test for trueness. If it wobbles, reheat the head enough to soften the glue, rotate the head a partial turn, and spin again. You may have to heat and spin several times to get the "sweet spot". If the taper is cut crooked, forget the broadhead and use the arrow with a field point.
On a difficult one, sometimes I rotate the head slowly enough to identify the angle, and press it to the side to correct it while warm. Doesn't always work, but worth trying on the hard ones.