Slimes spawn below level 40. You can use slime finders online but I just wait until I find some while mineing and cave exploring and mark that spot. I then go down to level 12 and excavate a room 3 blocks high. Slimes only spawn in about 10% of the chunks in the game and only about 10% of mob spawns in those chuncks are slime. So to increase slime spawning light everything up. Light levels do not affect slime spawn rates. But use jack-o-lanterns for light because slimes can spawn on them but not on torches. the large slimes need 3 blocks in which to spawn. The images are for my slime farm at the spawn area. If I'm in the area working on a house for a couple of hours then the slime farm will have 5 or 6 large slimes and a few medium and smalls.