Let's state the obvious first...the most common writers of love letters are women. We as women are more emotionally charged and because of that we find the most inspiration in quotes. I don't know of many women that haven't shared or sent at least ONE quote to someone they care about, be it friend, family, crush or love. When you are writing a letter to someone you admire the best thing to do is to stick to what you know. Many people make the mistake or writing something that they didn't come up with originally and use a sappy quote, just like the one below.
While it may seem like a good idea at the time, think back and see if those words really embody what you are trying to tell the one you love. Does it TRULY fit your situation as everyone is different. If you are using other peoples words, while it may seem romantic it can also appear tired or even fake, you never know who has sent that exact quote to the person you are writing to in the past. Do you really want to remind them of past memories? In today's day and age the internet could ruin any chance that you have with your crush/new love by recycling old tired quotes. If you think that your crush wouldn't like you because of what is running through your head, or you don't feel that you can put your thoughts onto paper, think again!
Your letter/message/note should be an original piece of work, something that comes from the heart. Think about what makes you happy, what you like about that person and write what YOU really feel. If you need to think about something else that makes you happy and relate it back to the person you are writing to then do it, whatever will help you to unlock that piece of originality that will set you apart from anyone else thinking about doing the same thing.
My parents used to write letters to each other all the time, my father is the LAST person you would expect to write a love letter and yet his letters make even ME cry because they truly came from the heart.
When you write your crush a letter remember to keep it short and to the point. Depending on how you know them, how much of an interaction you have had with them and what the topic is that you want to discuss, make sure that you don't make it something they will get bored of reading. Make sure that you check your spelling and grammar, I can honestly say that I hate reading something that has many mistakes (my own work included).
The best piece of advice I can offer is to write the letter, leave it 12 hours, come back to it and read it again. If you need to make any changes then wait another 12 hours and repeat the cycle. Only send it when you have NOTHING else to add to it. Then you know it's what you REALLY want to send, if you even want to send it anymore. So many people make the mistake of sending something they regret much later...myself included.
If you follow this basic guide, you will deliver the best note possible. Make sure you keep a track on what you have written, don't write the same thing more than once and switch up the tone as much as you can. Keep it light and fluffy and save the hard truths for face to face moments.