Hold the iPod firmly against a table or bench.
Lift the suction cup up and back toward the top of the iPod. Be patient, and pull with firm, constant force until the adhesive breaks and the display comes up from the rear case.
It may be necessary to use a heat gun to soften the adhesive (especially in cooler climates). If you can pry the screen apart, and the adhesive is still sticking and pulling apart like cheese topping on a pizza, you can slide a thin razor blade in and gently cut the adhesive.