[masterkeys]

Ok, start with the 18 inch, as others said, but try to find a sleeve bandage and wrap it around the body, for a tight fit, then as you get deeper slide move it down so you can use it to mark how far your lips got.
Once you start to gag stop, rest and see if you can take control of your breathing through your nose to see if reflex stops. If it does continue, if not try to hold as long as you can but don't vomit over dildo, keep a bucket or something nearby for that, and a bottle of minty mouthwash to rinse before retrying so you don't have vomit taste in mouth and don't get it on dildo either.
Then try again, as others have said try to last a minute to start, then once comfortably holding for a minute go deeper, and repeat until you can get all 18.