It seems that if there is ever a time to tell someone what they mean to you, it is on their special occasion, be it birthday, anniversary, or just to say thank you. As soon as you sit down with pen and paper, why do you get stuck? It may be that you feel stressed or under pressure to say the perfect thing. It may be that you are out of practice. Whatever the case there are some simple things that you can do to write a deeply personal, and therefore, individual message in a card or letter.
First of all, breathe, relax, stop thinking of yourself and focus on your special someone. Think of something about them that makes you feel happy, relaxed, even loved. Is it the twinkle in their eye, their smile, their laugh, is it their kindness, loyalty, or another attribute? Don't be afraid to tell them the thing you are thinking of, it will be a tremendous compliment.
Take a walk down memory lane - what is the first memory you have of this person? Remember a special moment or time in your life that you shared with this person. What is the funniest, naughtiest, or cheekiest thing you remember about your special person? Share your memories, memories bond us, if you share a memory your bond is strengthened.
If you are still short of filling your page, try sharing a quote, poem, or song lyrics, but always credit the original author. A poem does not have to be written by you to be appreciated - a beautiful quote or poem cited by you is still incredibly personal, it is flattering to think that these beautiful words reminded someone of yourself enough that they used it in a tribute to you.
I hope this helps you, I will have further tutorials along these lines in the future, my best wishes to you, Starry.