Getting a Max Serve requires you to hit the ball when the serve bar is at the top. This isn't too hard, though hitting 3 max serves in a row takes time.
I suggest you just play the game to see if it comes naturally. If you want to improve your chances:
Whenever you hit 2 max serves in a row, make sure you are able to do a third in the match. Basically this means:
- Do not win the point and the match after you make a second max serve, but let the other player win that point so you can go ahead an try for the third.
- If you make a second max serve on your game and ace it or win the point, let the other player win his serve game so you can try to max your first serve again in the final game (where you serve).
- Note that maxing a serve and faulting it at the same time DOES count towards this achievement. (that's how I got it).