1: Get its hp down to half ( at least )
2: Choose any kind of ball, even a pokeball will work(better to use a greatball)
3: When you throw it, hold up and "A"
4: When the ball opens up quickly switch from holding up and "A" to holding up and "B".
5: Hold these until it says you have caught the pokemon.

This code will take some practice but it works about 75% of the time.