How do i go about telling her?

i am enlisted in the Marines DEP and i leave June of next year. i have a very close fren who i know alot about and we have been frens for about 3-4 years. We are very close to eachother. that fren is a girl and i really like her more than a fren and she knows that, but she currently has a bf. i wanna express how much i will miss her and feel about her but i don't know how to go about it. It will have to be in person i know that.. but how could i bring true feelings out when i tell her? what should i tell her?