Do this while it is on the boat for best results. The lower needle valve screw on the front is for the high speed, and the one off to the side is for the idle. Before starting the motor, set them as follows. For the high speed, turn it all the way in and then back it out 1.5 turns. For the idle, turn it all the way in, and then back it out 1 turn. These are your initial settings prior to starting it on the boat.
Once you have it started, get it running wide open. Once it is running at WOT, start turning the high speed needle out (counterclockwise) in 1/4 turn adjustments, and give it a few seconds to adjust after each 1/4 turn. You should notice the speed improve. Continue to do 1/4-turns until it starts to "sneeze"; then bring it back clockwise 1/4 turn and it is set. Should run smoothly at WOT with the max HP.
Now, slow it down to a trolling speed and do the same process from above with the idle needle located on the side of the carburetor. Once it is idling smoothly, it is set. You should not need to adjust them anymore.
It is an easy process. Hope you gave the carburetor a good cleaning already. If not, I would make sure to do that before setting it.