Whenever you get a prisoner into solitary there is a small chance of him being able to become an informant you do this by going to any prisoner in solitary and pressing the prisoner options button which on ps4 is square not sure about xbox maybe the X button and the check if there is a recruit informant option if not select a different prisoner and keep doing this until a prisoner that goes into solitary has the recruit informant option but as i said not every prisoner in solitary has that option so you have to wait for a prisoner to go into solitary and allow you to select that option.
