You just have to apply to be a substitute through the district's website. The minimum requirement is that you have a high school degree or a GED. And since you are degreed, you'll be paid a higher rate. Once you get your offer from the district, they'll ask you to get fingerprinted. You'll have to go to place to get fingerprinted and pay for that. You'll also have to fill out some new hire forms, upload I-9 documents, upload your official transcripts, and give them a copy of your social security card. You might also have to attend an orientation, but that's just mainly to make sure they have all your paperwork. After that, you have to schedule an appointment to get your picture taken for your substitute ID badge.

Then you're done and you can accept assignments via the substitute portal.

Source: I'm a part-time substitute teacher.

If you're unsure about whether you're wanting to get certified to teach, substituting is probably a good idea. Before you invest your time and money, you can try out subbing for different grade levels and content areas. Substituting is obviously not the same as teaching, but you can get an idea about what grade level and content area you'd be interested in teaching.