I remember once in probably eighth grade, I was dating this boy who absolutely everyone I was friends with hated. They simply didn't approve of him. So, in order to remind him what he was worth to me, I took pen to paper and wrote him what (at the time) I thought was the most heart felt love letter that had ever been written.
Writing a love letter however doesn't need an occasion. It's simply a gesture you can choose to do to make someone you care about feel amazing, and to remind them of how much they mean to you.
But how do you effectively write a love letter? In all honesty it's as easy as sleeping or breathing.
You know that feeling they give you when they enter a room, and you turn to see them, and you see their smile. Tell them about that. It may seem strange, but that smile often gives you butterflies and makes you happy. So let them know how grateful you are for their wonderful smile.
Remember that time they made you breakfast in bed, but they burnt your toast, and how much it made you giggle like a school girl. Tell them that even though it didn't quite go to plan that it made you laugh and left you in a good mood.
It doesn't have to be a letter just full of compliments. Remind them of happy times you two have shared. Write about the little things they do for you, even if it is just washing your socks or giving you soup when you're sick. Not ready to write full blown love on paper? Then start with little sticky notes with a short sweet message like Your laugh always cheers me up.
Credit to Dragana Jokmanovic
And in the end, it truly is the thought that counts. Therefore write whatever you feel is necessary to make them feel good and truly loved.