From what my test book says in the vocab section the verb かきます usually takes the particles を for things and に for people. I'm pretty sure that you would use を like this.
ともだちに てがみを かきました - I wrote a letter to a friend/friends
So then I guess my question would be then for に would I just make the sentence like this?
ともだちに かきました - I wrote to a friend/friends(?)