1.first get the seed for the map you want to farm slimes on and enter the seed on this website.mcslimes.appspot.com/


2.dig out a 35x35 area once you have the coordinates of the slimes 4 blocks above bedrock level you can use tnt to dig out the area faster.


3.make sure the floors flat to increase the chance of them spawning.


4.light it up with torches not glowstone because slimes wont spawn on glowstone.


5.wait for the slime


please note this is only useful on xbox because on pc they now spawn in swamp biomes.