To get from Cabo Frio to Ilha Grande, you will be picked up from your hotel and taken to a meeting point where you join a shared transfer that goes to Rio de Janeiro. You will then take a new van which travels a few hours further south, and terminates at the pier where you will be provided a boat ticket for the crossing to Abraão on Ilha Grande.
For a faster ride, you can consider booking a private transfer from Cabo Frio to Ilha Grande which also includes your boat ticket. Abraão is the main population on the island and also where these transfers terminate. Because there are no cars on the island, you make your way from the main pier to your Ilha Grande hotel.