When your hull is complete, and your mast is tapered, mark the location for the base of your mast, depending on the sail pattern you will use to add sails to your boat. I am using a single mast and triangular sails, so I have set my mast roughly one third of the way back from the front of my boat.
Pick a mini drill bit the same size as the base of your mast. Center your mini drill squarely above your hole, at right angles to the deck (unless you want a raked mast) and drill a short hole into the center of your deck, roughly 1/8 inch deep. Test fit the base of your mast into the hole before proceeding further.