Start with a cobblestone generator. Then try to start farming. Look up tutorials and have fun!
Also try not to lose any dirt or items
I created a normal world, generated an 1000x1000ish area around the spawn point. Then I opened up the level.dat file with an nbt editor and changed the world generation settings so that it would generate as a superflat world for future generation. So now I had a void world with a 1000x1000 cube of normal blocks in the center. So I opened up MCEdit and deleted all the blocks in the ceneter. MCEdit only deletes the blocks, not the biomes or structure data. The final product is a void world with biomes and structure data.
The only issue is that the world save takes a while to download. It's a big file because of how much I had to generate normally.
1.) The world has normal biomes and structure data. That means the world doesn't generate any terrain, but Witch Hut and Ocean Monument spawning areas still form. For example, if you generate a world with normal terrain from the same seed (7990949358502160936) and find a Witch Hut, you can then build a Witch-Hut-farm at those same coordinates in the Skyblock world and witches will spawn instead of normal mobs.
IMPORTANT: The biomes/structure data is only "normal" within a 1000 x 1000 square centered on the spawn island. That area contains 1 Witch Hut, 1 Ocean Monument, and 1 cold biome (for ice and snow).
2.) The Nether is generated normally. Most skyblock worlds have an all-air Nether too. You could call this map a half-Skyblock world I suppose because the Overworld is all-air but the Nether has normal terrain. The player can farm nether fortress mobs and get essential items like nether quartz.
3.) The world has an End portal. The portal is in the same spot that it generates in a normal world with the same seed. This will be more important for playing in 1.9