How do you find slime on Minecraft?

First, you need to find a chunk which spawn slimes. To do that, dig to layer 40 and below and try to hear slime's jumping sound. If you can hear it, dig towards it and make a trap, with water directing the slime to it. You can just dig a 5x5x4 hole and put water directing to it, and you can kill them 1 by 1. Collect the slimeballs and make sticky pistons from it.