Obtain blaze rods by killing blazes in Nether fortresses
Craft blaze rods into blaze powder (fuel for brewing)
Obtain 4 obsidian blocks by mining obsidian with a diamond pickaxe
Create a brewing stand
Place the brewing stand and add blaze powder fuel
Add water bottle(s) to the brewing stand
Add nether wart to the water bottles to create awkward potion(s)
Add a ghast tear to create regeneration potion(s) (or other ghast-tear-based potion variants)
Add obsidian to the brewing process to create obsidian-related potion effects (via relevant recipes in your version/mod)
If using vanilla crafting only: mine obsidian with a diamond pickaxe; you cannot craft obsidian in vanilla