Ensure that the flat surface is clean (no dust) and dry. Affix the suction cup to the flat surface. Test that the suction is secure by tugging the toy. Apply a generous amount of lube to the head and shaft as well as yourself. Stroke the lube over the toy. Not only does it guarantee even distribution, it can be sexually arousing too. Mount, squat over or back up on the toy.
Most dildos with a suction cup are harness compatible; they can be used for strap-on play. (HYPERLINK strap-on)
Here is a list of locations where you can use a dildo with suction cup: