I have washed a regular keyboards in a dishwasher, electronics and all. Works fine; the downside is that it takes too long to dry. It can take several days before the last bits of moisture stop interfering with the membrane contacts.

Therefore, of course disassembly is the preferred approach. It is fairly simple for most cheap membrane keyboards, like from Dell or Logitech and whatever. You unscrew some dozen or so screws, and and then the top cover which houses the keycaps separates from the bottom. Removing the top reveals the silicone sheet that provides the rubber domes for the key action and protects the internals from spills and debris. That rubber sheet removes easily; it is just loosely placed over the membrane switch board.

You can wash the keycap housing in a dishwasher without popping the caps off at all. The rubber sheet can hand-washed in seconds in the sink with some soap and water.

That's pretty much it.