How do I migrate a question? The FAQ doesn't mention that. I don't even know if there are manuals somewhere.
It's an option you can vote on when you close a question as off-topic. You only get a few options, though. Moderators can migrate to any other site in the SE network, so usually you should flag a moderator for migrations you can't vote on yourself.
It should be presented as an option when you vote to close as off-topic.