Remove the old railings. Use your drill and remove all the screws that hold your railing to the boat.
Clean the area where the railing used to be. You want to make sure it's clean so the new railing can be installed flush against the boat.
Line up the new railing on the boat and hold it in place. You will need someone to hold the rail in place while you secure it.
Screw in all the screws that line up with the holes of the old railing. It is possible that all the holes will not line up.
Drill new holes to replace the holes that did not line up. Use the general-purpose drill bit sized to the hole of your screws, and drill new holes in the deck. Make sure your drill is positioned straight so your screws will fit straight in.
Install the screws on the new holes you opened on the deck. Use your drill to secure the screws in place.