How do I get a Special Issuance?
As with most specific medical questions, the only real answer here is "talk to an experienced AME, preferably one who's handled similar cases before". Some standard advice is to schedule a consultation only first, to review your case. Once you have everything organized and documented properly and your AME is confident about your application, then you can do the actual medical exam.

Having said all that, as far as I know you don't request an SI directly. If the AME determines that he can't issue a medical certificate because of a condition you have, he should work with you and the FAA to put together whatever combination of test results, documentation and general information that the FAA wants in order to issue an SI. But all of that should happen before you do the actual medical exam, to avoid delays.

If you're an AOPA member they can help you with advice and information and they have more details about the SI process. (Their medical forum is very useful too).