Sufficiently clever eldritch abominations should be able to reconstruct you from very little material.

• Your brain, of course, must be entirely destroyed.
• It's safer to destroy the rest of your nervous system, which might also contain information.
• Your genetic material and records of your actions (e.g. your comments on the Internet) are individually insufficient to deduce you, but I'm not so sure about the combination.

So first you want to erase as much information about yourself as you can. Take down everything you put on the Internet, burn everything you wrote, exert your right to delete personal information everywhere you have such a right.

You'll also want to distort other people's memories of you. (Or kill them too, but then we get recursive and reprehensible.) If you have sufficient time, you might do a few hugely out of character things and then isolate yourself completely. Maybe suggest a few false memories first.

There's probably nothing you can do about leaving DNA everywhere. At least try not to have kids.

Fire could work, but you're likely to burn incompletely. I suggest going out to a remote, hot area (think Amazonian jungle), obscuring your starting location as much as you can, going as far out as you can, and dying by having your head crushed or a bullet to the head. By the time someone notices your disappearance, figures out where you went, searches the area, and finds your body, you should have rotten completely.

If the eldritch abominations are coming right now and you don't have time for that, yeah, just jump into an incinerator. You should find one of these by following a garbage truck.

(Also, you okay, kid? This is just silly, Deep-Ones-dancing-on-the-head-of-a-pin musing, right? You can message me if you need to.)