Clean clean and clean some more. Inside the fitting, outside the pipe. Ream the inside of the pipe. Flux outside the pipe and inside the fitting. A light coat, not a snot ball. Assemble the joint. Heat the fitting not the pipe. when flux bubbles apply solder to the joint. 1/2" fittings need a 1/2" of solder, 3/4 -3/4 and so on until you get to 1-1/4" and larger which take more solder. Let cool naturally. Wipe the excess flux off the joint. If the fitting turns black when heating it's too hot. Disassemble it, re-clean it and start over.