Find an Ocean Monument
Defeat the Guardian and Elder Guardian mobs
Mine Sea Lantern blocks with a pickaxe to collect them
Craft Sea Lanterns using 5 Prismarine Shards and 4 Prismarine Crystals
Get Prismarine Shards by killing Guardians and Elder Guardians
Get Prismarine Crystals by killing Guardians and Elder Guardians
Use a Crafting Table to arrange the materials in a cross shape with one crystal in the center and shards around it