Use the 3M Green Fine Line #218 tape
I am right-handed and start at the port bow and work aft. I hold the tape at the water line with my left hand and pull (not stretch) the tape out about 24". Lay the tape down at the edge and work along. On the long straight sections of the hull laying out long pulls of the tape will eliminate the up-and-down wavering of 4-6" pulls.As you get to the stern, pull the tape out only 6" or so and continue to lay the edge of the tape at the water line. In order to make the curve, stretch the outside edge of the tape as you are laying the inside edge at the water line. Hold your left thumb against the tape where is is meeting the hull.Lift the tape back up if it is not perfect and place it back down again.This is easier if you do not have to work on a stepladder to reach the waterline.After doing the port side in one piece, I continue at the stern and work toward the bow.I use only two pieces of tape for the entire waterline unless I have a partial roll of tape.After all the tape is applied, I go around the entire length and press my fingernail along the edge of the tape that is against the waterline in order to obtain the tightest seal. I almost never get any bleeding under the tape.The biggest secret of success is the tape. Don't cheap out on the tape. The 3M Fine Line #218 is the best I've ever used. I use the 1/2" size for ease of following a curve. A 60yd roll is $6.55 at Jamestown Distributors. It is a one-day tape, but I have gone as long as two full days without problems.I pull the tape at a 30 degree angle AWAY from the painted section and 30 degrees UP from the surface of the hull. I remove the tape immediately after the last coat of bottom paint has been applied. The paint is set well because I paint the keel last and the waterline area has had at least an hour to dry.Don't wait a long time to remove the tape. If the paint is completely dry and hardened, it can chip the edge of the paint as the tape is removed. Remove the tape when the paint is still slightly soft.