Hardest part is finding a sponge...you can get one from defeating an Elder Guardian.
You can watch my video for "The Deep End" if you want to see how to find/defeat an Elder Guardian.
Once you have a sponge, get it wet by placing it in some water, pick it up, then put it into a hot furnace to dry it out.
All of my Minecraft HD Achievement Guides can be found here! :) (Mobile Friendly)https://www.bradleafsfan9.com/minecraft