Most states give multiple options for returning your ballot. You can mail it; drop it off in person, typically at an election office; or put it in a drop box. If you're mailing it back, do so as soon as possible to ensure it arrives before your state's deadline.
The most important thing to do is follow the instructions that come with your mail-in ballot carefully, including by using a signature similar to the one on your driver's license or voter registration. Then make sure you return your ballot on time.
This depends on the state, but many have trackers to see if officials have received your mail-in ballot. You may also be able to contact your county clerk to confirm receipt.