Mine kelp from ocean biomes (tall plants growing from the ocean floor) and collect 9 leaves worth. Dry the kelp leaves in a furnace using wood or coal. Take dried kelp to a workbench and create kelp block. Use kelp block in a furnace to cook food or dry more kelp leaves and achievement will be unlocked.
I've contributed to making a world/walkthrough earning you all achievements really easy. You can check out the walkthrough on the following link. https://www.trueachievements.com/gam.../walkthrough/1
The achievements are grouped per update, although if you are somewhat familiar with the game, just hop into the world and do the achievements without the help of a text guide. Please let me know if you need any help.