Mental mediumship is when one communicates on a one to one basis with a passed away person. Usually 3 people will be involved in a sitting the medium the passed away spirit and the one wanting to receive a message.
Here i will show you how to practice mental mediumship
1. Meditate for 15 min
2. Try and tune in to spirit world or heaven etc
3. Send out a message to who you want to talk to
4. LIsten for a reply (sometimes you will hear it like a thought popped into your head)
5. From there you can chat with the spirit
6. End the sitting by telling the spirit you are going away or for quickness say bye
If you get really good at it you will start to see the person (usually they might look younger or will show themselves as you remember them) behind your eyelids.
(sorry for grammer this was a quick mockup)