No need for preparation. It is almost impossible to prepare. Your bad mood can arrive suddenly or gradually.
Be in a bad mood.
Depending on your personality, if you tend to be in a negative state of mind, just let yourself be. Don’t try to cheer up. No need to be friendly, etc. If your regular personality is positive, you will need to make some effort to reduce your levels of positivity. The easiest way to do this is simply to not respond to situations which might trigger enthusiasm. Don't be close to nice people, ignore kind messages, etc.
In order to avoid real life problems, it is preferable to let people around you know that you are performing this work. Then keep on being in a bad mood.