Master Balls are extremely rare items that are found, not crafted.
Here's where you can typically find a Master Ball:
* Professor Elm's Lab: You'll find one in his lab very early in the game, before you even get your first Pokemon.
* Prize for winning a Contest: Occasionally, you might win a Master Ball as a prize from a contest, but this is very rare.
While you can't make a Master Ball, you can trade for one with another player if you're lucky!