Registering voters is handled differently state to state. In CA, it’s easy–anyone can do it. In Texas, you need to get certified by the state, precinct by precinct. Here’s a survey from the Brennan Center detailing it all. Fun fact up top:
At least 51 million — roughly 25 percent — of voting-age Americans are not registered and cannot vote. That is equivalent to losing the entire eligible voting population of California, New York, and Texas combined.