The title is the whole plan. Basket is back inside the Iron Island glass ring, pushing the dry area across, one section of water at a time. "We're doing some sanding."
A whole chest of sand
First stop is the sand supply. The chests up top have been tidied off camera, and one of them is packed with sand. "Oh my goodness, look at all the sand. That's amazing." The chest ring still has no labels, though. "I guess it's organized but not really."
johnfal_92 drops by
Partway through a quick tour of the base, johnfal_92 is standing up by the chests in diamond armor. In game chat, johnfal_92 points out that Resistance II should mean no drowning, and that with Respiration on a helmet "you do not take any drowning damage." That matters, because Basket spends most of the stream underwater saying "I'm drowning." johnfal_92 also thinks "we need a third beacon for symmetry." Basket counts it out: a third and a fourth, or maybe no beacons at all in the end. Then johnfal_92 has to go back to work and says "I will put in that beacon later," and logs off with "bye all!"
Walls, torches, sand
The method hasn't changed. Basket builds a little "fortification wall" of sand to hold back the water, fills the section in until the water has nowhere to go, then digs it out. Torches go down first so the sand falls onto them and breaks. Basket doubts the people who added sand ever pictured this use for it.
It isn't smooth. Currents keep pushing Basket out of place, the water updates drop the frame rate, and a catwalk turns out to be dirt and sand that Basket keeps digging up by accident. "Why did I make it out of sand, or out of dirt?" There's also a hole Basket can't fix yet for lack of blocks, and a surprise about the beacon: "I only have haste one. Who knew? What crazy person did that?"
Down to bedrock, eventually
Basket lays out the next step for anyone who wants to help. Under the chests, out to the back wall of blue glass, all the water comes out. Then, at the edge, everything gets cut straight down to bedrock, section by section, leaving the mob farms alone. "Keep in mind there's probably some lava down there so don't die." A chat message says two stacks of obsidian were left for Basket. A lot of the sand has already gone into glass, but there's enough to keep moving it from section to section.
Out of sand
Near the end Basket finishes the sand in the inventory, finds one more long stretch of water ("once we break that, that whole thing will go away"), and heads up to the very top to log off, out of the way of anyone working on the build. johnfal_92 joins the game again just as the stream wraps. The video ends on the credits, with no gameplay after them.