1. Wait for a few days after the last rain so that the taillights dry themselves, or remove them and dry them according to the previous recommendations.
2. Once the tailights are removed, wash off the gaskets.
3. Apply gasket sealant on BOTH sides of each gasket. You do not have to apply sealer to the bottom of the gaskets, I did that on purpose so incase it did fill up with water at least it had an escape.
4. Reinstall

If you use gasket sealer, you don't really need new gaskets. My gaskets were in bad shape, had the "cookie cutter" effect and also some tears around where the bolts go through.