Place a brewing stand and fuel it with blaze powder
Add a water bottle to the brewing stand
Brew the water bottle with Nether Wart to make an Awkward Potion
Add a fermented spider eye to the Awkward Potion to make a Potion of Weakness
Optional: add redstone to extend the duration
Optional: add gunpowder to make a Splash Potion of Weakness
Optional: add dragon’s breath to make a Lingering Potion of Weakness