Slimes only spawn in designated chunks on the Map. One chunk is a 16x16 area and it has to be in that area!
Another thing is that it DOES NOT have to be exactly down to bedrock they spawn anywhere from layer 16 and below.
IF there are any caves nearby light them up because if other mobs are spawning the Slimes wont a priority to the game and the mobs will spawn first (Hostile Mobs: Zombies, Creepers, Etc.)
It has to be lit up VERY well in order for them to spawn as well. You cannot kill them with a sword or any weapons, But you can kill the biggest naturally occurred one with a bow and arrow. Also make the ground flat because they will spawn easier because they wont fall into anything (because they only jump in a straight line)
they will spawn on Peacefull but it is kinda rare and the Higher the difficulty (for me at least) it seems like they spawn more often
If I forgot anything just message me and I will fix it. Thank and my Gt is I Own Trolling

I forgot to mention if you dig a 3x3 cube in regular stone and place Tnt in the second block it makes a perfect little cube that is a very efficient way to start the farm
And also try to use glowstone if you have it because it puts out more light than a torch and it also goes in 6 directions if you have it placed wright!!