It’s easy. Say Te extraño. Pronounced teh ex-trahn-nyo, te stands for you and extraño for first person of the verb extrañar, the infinitive form of the verb meaning to miss.
If you miss her very much, and I’m sure you do, you could add the word mucho (pronounced moo-choh) to the phrase – Te extraño mucho. I miss you very much.
Extrañar, by the way, also means to surprise, to wonder, as in That surprises me – Eso me extraña.