How to be a medium. Well, you are or you're not is the basic, quick answer to that. But there's a longer answer.
If you know that you have the ability to contact the dear departed spirits, then technically you are already a medium. A medium, though, is considered to be somebody who works as more of a trans-medium, puts themselves into an altered state to receive whoever may be, and I'm not being coy here, passing by.
That is literally what a medium is. It's a vessel, if you like, between here and the other side. It's the ability reach the other side and conver messages backwards and forwards, whether it's done formally in a circle or by one's self, that doesn't alter the fact. That is all mediumship,