When can a question be migrated, and who can migrate it?
Only questions which are less than 60 days old can be migrated; this rule applies also to moderators. Developers are able to bypass this restriction, but they only do so in very, very rare cases.
Any user with 3,000 reputation can vote to migrate a question to another site via a list of up to five sites, which are most relevant to that site. Simply load the close dialog, select the off-topic because close reason, and then select belongs on another site in the off-topic reason list. This will present the list of available migration targets.
On Stack Overflow, migration requires a super-majority when closing, which means that it will only be migrated automatically if four users vote to migrate it to the same site, on other sites, three votes for the same target suffice.
If you don't have 3,000 reputation, the site isn't in the list of available migration targets, or the question got closed without being migrated, you can flag the question for moderator attention and request that they migrate it for you. Simply explain in the reason which site you want it to be migrated. (Please select one site; don't list all possible sites it could go to.)
Things to consider when migrating questions
Don't migrate crap! Ever. If the question is likely to be closed on the destination site anyway, don't bother migrating it. Just close it normally and advise the OP of the other site in the comments. If they decide to improve the question, then they can go post it over there, or they can edit their question and have it migrated later.
Avoid migrating answered questions. The point of migration is to send the question to a place where it is on-topic and can get answered. If the OP already has an answer, then we've already defeated the purpose of migration and the destination site won't have anything to do with the question. Avoid migrating these questions unless they are of extremely good quality and risk deletion on their current site.
Don't migrate for the sake of migration. We only migrate questions because they are off-topic on the original site. It is perfectly possible for a question to be on-topic on multiple sites, but that is not a reason to migrate it elsewhere. As a general rule, if someone asks a question here, and it's on-topic here, it should stay here.
Be familiar with the destination site. If you have never even visited the destination site before, read through the FAQ thoroughly and make sure the question actually belongs there. Don't look at the name of a site and automatically assume the question can be asked there. Each site has its own rules, and rules vary greatly across sites.
If you're not sure, don't migrate it. Let someone else who is sure do that, or ask for opinions from the community in a relevant chat room.
What causes a migration to be rejected and what happens after?
A migration can be automatically rejected before it even gets migrated if any of the following conditions apply:
- The user is question-banned or suspended on the destination site
- The question does not contain any tags that exist on the destination site (and the destination site isn't a meta site)
- The question is an exact duplicate of another question on the destination site
A question can also be rejected by the target community after it has already been migrated if it gets closed as a reason other than duplicate, or gets deleted (usually by the owner) on the target site. When a question which was already migrated gets rejected, the entire process of the migration is reversed. All the answers are returned to the origin site and undeleted* and are subsequently deleted on the target site.
*Note: If the question was already deleted on the origin site, migration rejection does not cause the question to be undeleted.
This answer has been copied verbatim from Meta Stack Exchange.