To avoid confusion while on the water, don't position the numbers front to back of each other. If you have a "3" on your sail number, for example, it will look like an "8" to other sailors. By convention, the starboard
sail numbers are mounted higher, port
sail numbers lower.
Lay your sail out on a clean, hard floor. Use a strip of masking tape to create a straight line as a reference for the bottom edge of the line of sail numbers. Position the numbers and tape in place with a single short length of masking tape at the top of each number. Hinge the number up and peal down a small amount of backing paper. Hinge the number back down again and begin sticking the number to the sail. Being careful to maintain proper number position, continue to peal additional backing paper off of the number as you adhere more and more of the number to the sail.
When you have all or the numbers installed on this side of the sail, turn the sail to the other side. Use a stiff item like the edge of a credit card to burnish the numbers onto the cloth. Rub the edge of the card over the cloth as you view the numbers through the sail cloth. You can see your progress as you work. As you burnish the numbers down, the adhesive will bond to the fabric and the appearance will change a bit. You will be able to tell when you have the entire number properly burnished down.
Repeat the process for the numbers on the other side of the sail. The entire process is harder to explain than it is to complete!