Either Alone or with a friend in your party, start practive VS AI.

Once in the practice, edit the game and remove ALL AI. To farm slaps quickly it should be 1v1 so you can easily keep the score tied if you are close to 5 slaps.

Chance the court size to SMALL. This is important as you can't hit it against the far wall in a normal or large court off the server

Once you are in the game whoever gets serve first starts to attempt the farm.

The most successful way is to line up a straight server that goes over the net, off the far wall and back setting you up for an easy slap. It takes practice to get this server going consistently. Once you do, make sure to NOT hit the 3 point circle as it could end the game too quickly.

If you screw it up, it is better to take a point and allow your friend to serve and try their luck. Keeping the score close is key to get a good farm without having to start new games all the time.

If you are not at 4 slaps by game point restart the match. If you are, make sure the score is tied before trying for another slap. If it is not tied, you will end the game early and you would need to start again.