Goto Ships -> Manage Piloted ships
Select the ships to NPC pilot by clicking the box next to it.
In the drop down box select 'Set Controller'
Select 'NPC controlled' and press 'apply'
Obviously you dont set the ship you are flying to NPC controlled. The only other option is to set as unmanned.
The criteria for which ships you can NPC is simple. They have to be of a type which are NPC'able (see rules NPC's) and be at your location and assigned to or owned by you.