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Carefully slice the tip off to make a hole that you can squeeze the caulk out of. Make sure to use silicone bathroom/kitchen/plumbing caulk for your drain to ensure it is waterproof. You can get a tube of silicone plumbing caulk at any home improvement store. Put the tip of the caulk tube in the end of the caulk gun furthest from the handle. The rest of the tube will snap easily into place, as long as the tube and the gun are the same size. You can get a caulk gun that fits your caulk tube at a home improvement store. Hold the caulk gun at a 45-degree angle over a scrap piece of paper. Pull the trigger until caulk comes out in a steady stream. This will ensure that you can get an even bead of caulk out around the drain. Hold the tip of the caulk gun at a 45-degree angle to the rim of the drain hole. Squeeze the trigger and slowly move the tip of the caulk gun around the opening until there is an even line of caulk all the way around. If the caulk stops coming out at any point, then release the trigger and press it again until the caulk starts coming out again. Some caulk will squeeze out from under the lip of the drain body. This will create the waterproof seal around the drain. If no caulk oozes out after you tighten the drain body, then you should remove it and apply a bigger bead of caulk to make sure you create a tight seal. Thoroughly wipe away the caulk that oozes out around the drain after you have tightened it. Throw away the dirty paper towel. Try moistening the paper towel to wipe away the caulk if it is hard to get all of it off with a dry paper towel. Put the tip of the screwdriver against one of the nubs with screw holes inside the drain body. Hit the top of the screwdriver with a hammer to tighten the drain body about 1/2 a turn more to complete the seal. You can also use some kind of utensil, like pliers, to wedge inside the drain body against the screw hole nubs and tighten it another 1/2 a turn. Align the grid so the screw holes are over the nubs inside the drain body. Place the screws in the holes and tighten them all the way clockwise with a screwdriver. Be careful not to drop the screws down the drain when you are putting them back in!
Cut the tip off a tube of caulk with a utility knife. Insert the tip of the caulk tube into a caulk gun and snap it in place. Pull the trigger a few times to prime the tube. Squeeze a bead of caulk around the drain opening. Screw the drain body back into the pipe and hand tighten it as far as you can. Wipe away the excess caulk with a paper towel. Tap the drain clockwise with a screwdriver and hammer to tighten it all the way. Replace the drain cover on the drain and screw it down tightly.