Mark Mandell - Gone Round In Jersey
Given that you've properly cleaned and "greased" both the pipe ends and the valve that is put in the open position . . .

Apply your heat such that the flame cone is on the lip of the valve, but pointing away from the center of the casting. Move your flame around the casting to evenly heat the brass. Keeping the wire solder out of the flame, apply to the pipe/valve joint on the top surface allowing the metal to melt the tin down into the joint. When you get a drip starting at the bottom, flick it off with your rag, then reverse the torch and repeat at the other end.

Remember, only the solder inside the pipe/valve joint makes the seal. You can wipe a pound of solder around the outside and the joint will still leak if the pipe is not fully inserted into the casting with the joint properly filled. Remember to, the solder melts and runs toward the hottest point.