You need a Brewing Stand
You need Blaze Powder
You need Water Bottles
You need Nether Wart
You need a Fermented Spider Eye
Put Blaze Powder in the Brewing Stand fuel slot
Place Water Bottles in the bottom slots
Brew the Water Bottles with Nether Wart to make Awkward Potion
Brew the Awkward Potion with a Fermented Spider Eye to make Potion of Harming
To make Potion of Harming II, brew the Potion of Harming with Glowstone Dust
To make Splash Potion of Harming, brew the Potion of Harming with Gunpowder