Focus your love letter in on what makes them so special and unique. Do not compare them to other people. Especially do not compare them to people from past relationships.
Focus your love letter on how pleasant they make you feel when they are near. Tell them about all that you notice about them - like how that person hair looks, or how you enjoy the way that the person smiles. Having a photo or two in front of you will make this easier. Find a quiet place to write.
The purpose of a love letter is twofold. One to express your love and the second to win over the heart of a person. The more that a person has positive feeling when they encounter you, the more like you are to make them love you. Your love letter should always make that person feel special and important. A nicely crafted hand written letter will go a long way in today's society of mass production and electronic communication.
Use a good quality stationary for the final draft or some special romantic paper. Never use lined paper or computer paper.
If you have trouble writing on stationary, place a piece of lined paper underneath the paper to give you guides.