Things You'll Need
Instructions
Locate a water source and fill your bucket with water.
Find a cave. Dig a hole near the bottom of the cave that measures at least two blocks by two blocks to ensure that a slime of any size will fit inside. The depth will vary depending on the size of the slime and your intentions. Small slimes will drown in a water-filled hole that's only one block deep. However, if you want to keep larger slimes enclosed, you can make a hole that is two blocks deep. If you want to kill the big slimes, make it three blocks deep so that the water goes over their heads.
Use the bucket to fill the hole with water.
Wait for a slime to spawn. When it does, get close enough so it notices you and then lead it to the water hole.
Stand on the opposite side of the hole from the slime. It will attempt to cross the water to get to you and fall in. If you left the hole shallow enough, and if the slime is big enough, it will remain trapped in the hole. Otherwise, it will drown.