Ferrell's theory stated that it was actually possible to communicate with spirits through objects that could've had a greater meaning on the spirit's life or an object directly related to such, which is now a commonly accepted theory among the people who believe in this kind of 'afterlife'. 

Once selected the item, it would establish some form of 'link' between the person and the spirit, the spirit being able to tamper with said object as it desired. On Jonathan's case, he chose to use his dead wife's diary, he'd often write questions for her and get answers, in order for Ferrell's experiment to work, he kept the diary a secret and told no one about it's location or existence, it was later revealed in Jonathan's documentations that he hid the journal inside the walls of their own house. 

Though there are very little found pages of Ferrell's diary, I was able to find some official ones on a few forums, hand-writting checks out. This is the most reliable known instance of a communication between a ghost and a human. Jonathan's theory has been tested multiple times along the years and witness claim this method is legit.

However, it's still worth mentioning this method is also claimed to be extremely dangerous if done properly. Multiple theories and studies claim the Un-Dead can't interact with the physical world, at least not in any significant way. By doing this process of communication you're allowing spirits to 'enter' the physical world through objects, which could lead to some very awful consequences.

There are many other methods of contacting ghosts, though, obviously, there's not enough information to back it up for me to write about it in detail.