You book your snorkeling outing at the Sports desk - usually book a couple of days ahead. Plan to snorkel earlier in your stay - if it's too windy, they may cancel the outing and you'll have to rebook so don't wait until your last day or two.
They have all the equipment and they provide a tutorial in the swim-up bar pool about an hour before you go out (if you need it). They have a couple of sites they go to. We snorkeled at the reef just east of the resort. It's a great time - have fun!