https://www.youtube.com/watch?v=Y9gZ6TmDz-g&t=6s
You are probably looking for something like this.
- Turn petcock to off
- Disconnect your fuel line from your carburetor and extend it somehow to a container.
- Turn your petcock to reserve (that way you will train the entire fuel)
It might take some time to drain the fuel. Leave it be and check occasionally. There should also be a way to drain it fast since I have heard of people stealing gas from motorbikes but I don't know how they do it.
This won't work if the bike is injected or your "petcock is vacuum operated". I personally am not a carburetor type of guy so you should cross-check this answer.(I would appreciate if a more experienced member could confirm or fix if necessary)