This would be as an independent candidate of course.
To “run” you don’t really need to register anywhere. You’re free to go out tomorrow and say “I’m borax, and I’m running for president. Please vote for me.”. If you aren’t constitutionally qualified, that would be a singularly useless exercise, of course. Presuming you are qualified, it’s still effectively a useless exercise because you aren’t on any state ballots, and would be asking people to vote for you as a write-in candidate. The rules for getting on the ballot vary from state to state, and you need enough organization behind you to deal with the fifty-one (I’m assuming you wanted on the ballot in DC) individual procedures. Many minor parties do not get their candidate on the ballot in all states.
Here’s the process for New York, for example. To summarize, you need 15,000 valid signatures on a petition, including at least 100 from half of the congressional districts. (In other words, NY has 31 districts, you need at least 100 signatures each in at least 16 different districts.)
Then you must comply with the financial disclosure laws, which I believe exist on both the federal ans the state level.
None of this is a trivial process.