Look for a small, usually rectangular or cylindrical valve with a vacuum line connected to it. It's often near the back of the intake manifold, somewhat towards the passenger side (right side in a right-hand-drive vehicle), but not always easily accessible. You'll probably need to get a good view from underneath or potentially remove some components for better access.
A service manual specific to your year and model Ford 5.8L is your best resource for precise location. Pictures in the manual will help significantly.