How do you become a write in candidate to be president?

You do not have to anything to be a candidate- just announce that you are a candidate. Most, if not all, states allow write-in votes, so you can write your own name in and not only be a candidate but even get some votes from your relatives and friends .

To get on the ballot you have to file a proper petition with some large number of signatures of registered voters in the state and submit it by the announced deadline. You have to file a separate petition in each state and be sure to follow all their laws governing such petitions. To this requires a national party organization to help you.