Step 1) Make sure nobody's around.
Step 2) Quietly open the lid of the upper-section of the victim's toilet.
Step 3) Quietly place the lid down on the floor.
Step 4) Then take a nice ♥♥♥♥ in the upper-section of the toilet without letting anybody hear you.
Step 5) Wipe your ass.
Step 6) Place the soiled toilet paper in the upper-section of the toilet or in a drawer or magazine.
Step 7) Slowly lift the lid off the floor with your fingers under it and carefully bring it over to the toilet.
Step 8) Here's the hard part; with your fingers under the lid
slowly place the lid on the toilet. No false moves or you're screwed.
Step 9) Leave and don't let anybody see you. Just bail as quickly as possible.
Step 10) Mission accomplished.

Plumber: It looks like you have feces in your toilet's tank.
Victim: It must have been an upper decker.
Plumber: Yeah right. Its already clear that you're stupid enough to ♥♥♥♥ in there!