To get there you have to take one of the boats (private or collective) that leave from the port of Chiquilá, located in the northern tip of Quintana Roo state. It will take you about 20 minutes to cross the Yalahau lagoon to reach the island.
In addition, Vacaciones Barceló offers you a day tour to Holbox (departing from Cancun and Riviera Maya), which includes, of course, the transfer to the island, among other services.