Minecraft Skyblock building down in nether
Solution 1:
You have 3 options in my opinion:
1)Acquire a piston. With this piston you can push blocks down. This can be repeated as many times as necessary. (Iron can be obtained by waiting for a iron bar drop from a zombie in a mob-spawner.)
2)Have a watercourse near your over-world portal. Create a down flowing lava-fall and drop down in this to build. Quickly return to the over-world and put out the fire. Rinse and repeat.
3)Plunging to your death whilst placing blocks on the lowest level of your island.
Solution 2:
simple solution: piston pushing the cobblestone down.
less simple solution requires the witch to throw fire resistance on herself, you being affected because of standing near her, than running through the portal and lava-scaffolding.
Solution 3:
You are able to get pistons before getting a villager. You are able to get Iron from killing zombies, but the redstone is a bit trickier. The villager method is definitely the best way to do it, but there is also the possibility of finding a place where a witch is supposed to spawn (witch hut) and kill witches for it. They spawn naturally without huts, but it is quite rare, so that is another option. It shouldn't be too hard to get 1 redstone this way.
As for powering the piston, a button works, which is very cheap. You push the blocks down little by little, then you can make a stone platform to catch everything you need. Good luck to anyone playing now, Skyblock is an underrated game mode. Remember, Skyblock for 1.16 has recently come out, so check it out!