How do I disassemble my Canon 18-55mm lens to repair water damage inside it?

Given that you’re talking water damage, and that the lens is probably already trashed, I’m not sure you can do any more harm to the lens if it’s not working at all. An 18-55 kit is cheap enough that nearly anything totals it. Just be sure that you’ve made peace with the fact that you’re just as likely, (or more, if you’re inexperienced with electronics and lens repair) to destroy your lens as fix it, before trying to crack it open. Have an alternate plan in place (maybe replacing with a used one?) if you destroy the lens. The odds are against you successfully repairing it.

You may also probably have to expend some money on appropriate tools. Most folks tend not to have spudgers, 00 screwdrivers, plastic tweezers, and spanning wrenches lying around, although among this SE crowd, maybe not. 🙂

Here’s a youtube video that demonstrates full disassembly. Hopefully your version of the 18-55 is similar.

It looks like after removing all the screws from the rear assembly (there are a couple on the sides), the back should just lift out, but will be connected by a cable that you have to unplug from the board, so be gentle.

Further screws hold down all the other boards, and there are a number of cables that have to disconnected. And you will also have to go in through the front of the lens, which will require prying up the trim ring with a spudger to access the three screws holding the front element in place, and removing the rubber grip ring to get to the focus helicoid screws.

Good luck on getting it all together again in working order!