Craft a Brewing Stand
Get Blaze Powder to fuel the Brewing Stand
Get a Water Bottle
Get a Fermented Spider Eye
Place the Water Bottle in the Brewing Stand
Add Blaze Powder as fuel
Put the Fermented Spider Eye into the top slot
Wait for the brewing process to finish
Collect the Potion of Weakness
Optional: Add Redstone to extend the duration
Optional: Use Gunpowder to make it a Splash Potion of Weakness