Faq says that you need 250 points to be able to vote for migration of questions, but still I don't see how to do that.
250 is the minimum reputation you need to vote to close/migrate your own questions. On Stack Overflow you have a reputation of 3000 necessary to vote to close/migrate any question.
If you want to migrate your own question, you click on close. (I took the screenshots on Stack Overflow on March 19, 2018.) On the dialog box that appears, click on off-topic because....
In the next dialog box, click on This question belongs on another site in the Stack Exchange network.
Then, on the next dialog box, you click on the site you want to migrate the question.
As per March 20018, Software Engineering is not listed as possible migration path. In this case, you could click on flag, and then report the reason to flag the question (e.g. The question better suits XYZ).
If you find a question asked from somebody else that needs to be migrated, then you click on flag, then on in need of moderator intervention, and report the reason you are flagging it.
This is still the way to report a question needs to be migrated to a site that is not reported in the list you see when you select off topic.
Notice also that it's not ANYMORE possible to migrate questions asked more than 60 days ago. In this case, clicking on close would give you the following dialog box.
As you see, the off topic because... option says (too old to migrate). If then you click on that option, the next dialog would not show the This question belongs on another site in the Stack Exchange network option.
In this case, flagging the question doesn't get any effect: Moderators would not be able to migrate the question, which can only be closed, if it's really off-topic for the site where it was asked, or if it is really a bad question.
What I reported is valid for the reputation you have on Stack Overflow. Once your reputation is at least 3000, you can vote to close any question; in that case, you don't need to flag a question to migrate it, except when the site where you think the question should be migrated is not listed under "off topic."