Sorry for this simple question, but I just did not find out how I could add a bounty to one of my questions. (Maybe I just do not meet a requirement for adding a bounty, but I did not find out about that either).
The main requirement I know of is a time requirement: you have to wait something like two days after you post a question before you can start a bounty on it. When enough time has elapsed, go to to the question's page. Just below "Add a comment" and just above any answers that you may already have received, it should say "Start a bounty". Click on that!