Dinghies that will be raced are required to be clearly identified with their Sail Number. Many people learning to sail start out having no interest in or intention to race. However as confidence increases with skill levels this often changes.  The fact of being in a race causes many of the sailing techniques previously practiced when cruising to gel and simply being in a race, regardless of your skills, quickly illustrates areas of your sailing techniques that can be improved, to say nothing of the enjoyment of the race. When you first start racing do so for the experience and enjoyment.
The standard Xenon is fitted with a Mylar main sail that when supplied does not have sail numbers (Topaz Xenons used in the Endeavour Trophy race come ready with sail numbers - these can be identified by the white gennaker). The sail number of the Xenon can be found on the Topper manufacturers label fastened inside the dinghy. Incidentally the sail number bears no relation to the numbers of dinghies produced.
You will need to purchase suitable sail digits, one set for each side of the sail. Each digit costs around £2 and consists of a nylon self adhesive type of material with a waxy backing paper.  Sail numbers are available from several suppliers. I purchased ours from Pinnel and Bax who offer different sizes and font types.  The Xenon has a large sail and 12" (30cm) high digits are both clear and look proportionate on the large Xenon mainsail.  A 50mm wide roll of masking tape and a red or green whiteboard marker (easily removed from mylar sails) will also help in aligning the digits.
Attach the numbers to the panel below the lowest full batten, ie the panel below the Xenon logo. Attaching the numbers neatly is not quite as simple as it may first seem - the adhesive is strong and you only get one chance to position the digits on the Mylar surface. You may find the following advice helpful, but there are no rules about the positioning as long as the number is clearly displayed. Many dinghies display the thousand units using red digits, the remainder being in black. With transparent sails this a helpful practice because the red colour also indicates the start of the number and avoids possible confusion caused to race officers by the reverse outline of the number on the other side of the sail. The numbers look best if positioned equidistant from the leech and luff of the sail. By convention the upper sail number should be on the Starboard side of the sail (WB-Sails sailmakers). Place the sail flat on the floor or large table, starboard side uppermost, and put a piece of board under the sail to provide a firm base on which to rub the self adhesive numbers to make sure they are stuck firmly and free of air bubbles.
Start the aligning process by sticking a strip of 50mm masking tape at a distance of 20" or 50cm below and parallel to the lowest full batten. This tape will mark the lower boundary of the sail number on the starboard side of the sail and the upper boundary of the number on the reverse side of the sail. On the first side, experiment by laying the numbers out using the top of the tape to keep them straight, experiment with the spacing until you are satisfied with the appearance. Aid the spacing by marking the masking tape at the point where the digit will touch the tape. Take care to ensure the digits are straight and not looking drunk. Hold the first digit in place and using the whiteboard marker draw around its outline, or at least as much as is necessary to ensure it is in the correct position.
Start the sticking operation by peeling back a small section of the backing paper from the part of the digit that will be aligned with the masking tape. The adhesive is strong and will fasten to the mylar on the slightest touch so take care not to touch down on the mylar until you are sure of the correct position. Check against your marker outline and when satisfied carefully and lightly touch down. Once pressed down it is very difficult to remove a digit. If you make a mistake pull it up immediately before the digit has been pressed down. if necessary enlist the help of another person as another set of fingers can be useful.  Slowly peel off the backing section by section and rub down taking care not to trap air bubbles under the digit.
Position the next digit at the mark made on the masking tape - this defines the spacing - then use the whiteboard pen to outline the digit after checking for drunkeness. Stick down as above. Repeat the operation for each digit.
After completing the number on the first side firmly rub down all the edges before starting on the remaining digits.
Leave the masking tape in position and turn the sail over. On this side use the bottom edge of the masking tape to align the top edge of the digits. Once again check the spacing of the digits and use the whiteboard marker to draw the positions on the mylar sail (the marks can easily be rubbed off the sail when work is complete). Stick down the digits as before. Firmly press down all digits and use a cloth to remove the whiteboard marker outlines.
PS  2 years afterwards our sail numbers are still very clear and very firmly attached.  Enjoy your racing!