Gather 1 Water Bottle
Gather 1 Nether Wart
Gather 1 Pufferfish
Gather 1 Brewing Stand
Gather 1 Blaze Powder
Put Blaze Powder in the Brewing Stand fuel slot
Place the Water Bottle in a brewing slot
Brew it with Nether Wart to make an Awkward Potion
Place the Awkward Potion back in the brewing slot
Brew it with Pufferfish to make a Potion of Water Breathing