This is a painstakingly long process. It requires a lot of patience. Things you'll need: Lots and lots of a building block of your preference. A lot of torches. Lots of time, and patience. Because the seed isn't given nor acquirable you need to go about this the hard way. 1. Build a large platform under your skyblock below y:40. I'd recommend using half-slabs because they're much easier to obtain. I made my platform 40x40 2. Light the thing up. Make sure you're prepared and bring a lot of torches. 3. Look for slimes. If you're lucky at least one will spawn if not you'll have to expand. Easiest way to tell if there are slimes is to just leave your game on, at your island spawn, for about 10 minutes, then go back down and check. 4. Chances are they didn't spawn in the middle of the platform. To create the farm accurately you need to see the slime spawn. It won't move for a couple seconds, it's just to determine the block it spawns on. After that you need to measure the chunk which the block is in. Do so by mapping out the (x) and (z) coords and checking if they are divisible by 16. 5. If everything is right, then you should have slimes spawning. Make a fence or wall around the spawn chunk I'd recommend adding a glowstone grid like ceiling 3 or 4 blocks high to make sure the place is well lit. 6. That's it. If you have any problems with this method just post it here.