KUSA — I've been doing this for decades. Each Thanksgiving, I save a big pumpkin to make a centerpiece.
Cut open the top, just as you would to carve a Jack-o-lantern. Scoop out the seeds. Insert a jar or vase that fits the opening. (Plan ahead before you cut.)
I use greenery from the garden to fill in and spill over. Then I insert flowers from an inexpensive supermarket bouquet. These are usually quite well done; I just trim three or four inches off the bottom stems. Just plop them in. I usually buy an extra bouquet of mini carnation, just in case. If I have any gaps, I fill in with those.
The whole arrangement (except for the vase) is biodegradable. Set the pumpkin out for the wildlife to enjoy and compost the flowers.