1. Pick something that is in your head regularly.
2. Every time you find yourself thinking about it, redirect your mind. Don't get stuck on how long it takes you to redirect your mind. Just do it as soon as you catch yourself.
3. Notice the difference in your mind after doing this for a while. Is your mind still bugging you about that thing?
4. If not, can you try it with the new thing that's coming up?

This process is continual. The mind loves to get stuck in thought loops, especially in relation to problems it can't immediately solve. The more you redirect your mind, the less likely you are to get stuck in those thought loops. The mind will quiet down. It just takes time.