-Just make her feel like she's the only girl in the world.
-Let her know that she's more beautiful without make up.
-Let her know that she looks absolutely amazing in sweatpants more than she any other outfit.
-Let her know that you would never want any other girl in the world, and make sure you mean it!
-Don't ever talk about ex girlfriends.
-Tell her how much you love her, ALL THE TIME, even when you argue.
-Make it so she wakes up to a nice goodmorning text from you.
-Be there for her.
Let her know that you're her best friend, and you don't know what you'd do without her.
Before you do any of these things, make sure you actually love her,
make sure your feelings for her a real, because if they aren't and you
tell her these, she's going to end up with  broken heart, because
whether you believe it or not, she will find out that you 
didn't mean any of it at one point, and will be broken hearted.
Just make her feel,

beautiful♥