Separate keyboard film stickers are very easy to use. I've tested them personally.

These keycaps have been worn down from long-term use. They look oily, but they're actually not. There is no way to fix this, you can only replace the keycaps.

I use alcohol wipes for glasses, and I wipe my keyboard with them after I'm done cleaning my glasses every day.
This is a great way to keep your keyboard clean and free of oil and dirt. Alcohol wipes are also very effective at killing bacteria.