Getting an ASOPRS plastics spot is like catching a hail mary. I dont think there is any algorithm to follow to obtain a spot. But in general going to a strong residency with a great oculoplastics department, high OKAPS scores, a strong research background and likeable/hardworking personality will all help.
I think though the most important is the oculoplastics faculty at your program who will ultimately go to bat for you with strong recs and phonecalls. This though is usually earned by demonstrating the other factors I listed above.
Keep in mind that the application and interview process for oculoplastics occurs during the second year of your ophtho residency (i.e. PGY-3 year) and so you really need to get involved early to build your application.