He tore off Georgie's arm at the shoulder by using his paper boat to lure him in closer, and when the screaming kid tried to crawl away in the rain, calling for his brother (a MC of IT), Pennywise grabbed him by the ankles and dragged him into the sewer system.
Pennywise ate (or most of) his body, and like all his other victims, Georgie's soul was trapped in the Deadlights (Pennywise's true form) and tortured.
Ironically, Pennywise IIRC was actually after Bill, but he had to settle for Georgie, instead. And, double ironically, if he hadn't had Georgie as a snack, the series of events that led to Pennywise's defeats and ultimate death (debatable, as other King novels had cameos of "Pennywise Lives!" and the like after the events of IT) wouldn't have happened -- he'd still be munching on kid-meat and hibernating in ad finitum to this day.
Here's a funny scene: the actor who played Georgie was actually scared of the actor who played Pennywise, so they did this funny scene to calm/ease the kid for when they did the real scene (Georgie being caught, maimed, and eaten).
