Create or find a Crimson world
Defeat the Brain of Cthulhu
Open the Crates dropped by Creepers during the fight
Collect Tissue Samples from the Brain of Cthulhu and Creepers
Mine Crimtane Ore and use it to summon the Brain of Cthulhu again if needed
Craft a Bloody Spine at a Demon Altar using Vertebrae and use it to summon the Brain of Cthulhu
Farm the Brain of Cthulhu repeatedly for more Tissue Samples