Slimes spawn underground at high altitudes, so digging a hole in the ground near tall mountains will increase the chance of them spawning.
There are two ways to track slimes down. One is by asking villagers if there have been any sightings nearby, and the other option is by keeping your eyes peeled as you travel around in deep caves or jungles. When skybaxes (slime) fly from one high altitude spawning point to another they will always stop midway for a short time; this makes it possible to catch them more easily just like with fish since they’ll be visible beforehand and return back “home”. To make things even easier, minecraft has quantum-entangled slime blocks.