How can you summon a spirit?

You can perform a Seance to communicate with the ghost, which basically means you have to have the ability of Necromancy.

Answer

First we have to establish just what a ghost is. Would it surprised you to know that there are a number of ghosts who appear in different forms. Let us first discuss the ghost which Satan conjures up. These are evil spirits which enter the body of a person who desires to serve Satan. Their bodies may be inhabited by the spirit of an evil spirit who resides with Satan.
Others are Ghost or spirits who are worthy people who have passed on, but are given some assignment to visit the earth, either to do the will of their father in Heaven by causing some calamity to warn the people, or who are to give some message to some living person.
There are three ways in which you can tell where these ghosts come from. A ghost, if from Satan, when asked to shake their hand your hand will pass through theirs. A righteous ghost will refuse but give their massage, while the other will be a resurrected being and so you will feel their hand.
Spirits can be asked to visit with you if your reason warrents it and God or Satan sanction it.


Another Answer:

Within the depths of our minds are all the spirits one could wish for. They are the archetypes that represent various desires, fears and actions. One can use a black mirror in the dark to distort ones reflection into becoming one of these archetype spirits.