You can do this by touching the stamen of the male flower to the stigma of the female flower. The stamen is in the center of the male flower and is yellow from the pollen. The stigma is in the center of the female flower and is sticky. The flowers will rot away and your pumpkins will begin to get big. Fertilize them with plant food through the summer and pick the pumpkins when they are the size you want.
Pumpkins can get quite big.
If you want to know more about giant pumpkins, go to How To Grow a Giant Pumpkin I tried growing pumpkins, but mine didn't grow because we did it all wrong. I didn't rake the dirt or fertilize the soil. I just threw the pumpkin seeds on the ground. I did this at the wrong time of year in September. It was an experiment.