Measure the circumference of the top of your pot (mine was 9 inches) and using a sharp knife cut a corresponding circle in the top of the pumpkin. Since my flowerpot is smaller than my pumpkin, I made my cuts with a sligtly slanted edge so that the pot would balance on the pumpkin. You could also put newspapers or some other filler inside the pumpkin to bring your pot to the correct height.
Carvable pumkin (Michael's)
Potted plant (Home Depot)
Knife, measuring tape, pencil for marking cuts on pumpkin
Time spent: 15 minutes
Difficulty: Very easy
Also shown in bottom photo: Mums in jack-o-lantern pot (both from Home Depot), small sugar pumpkin (Cox Farms) and wooden outdoor table (rescued).