Start with sugar pumpkins
They are usually pretty small compared to the jack-o'-lantern varieties. The ones I found were between 2 and 3 pounds.
Wash and peel the pumpkins
Use a vegetable peeler to peel away the pumpkin skin. The stem of the pumpkin makes a nice handle as you work.
Scoop out the seeds and stringy stuff
A melon baller or a regular spoon does the job.
Puree the cooked pumpkin
After about 30 minutes, the pumpkin cubes should be very tender and ready to be pureed. Using an immersion blender is the easiest way to go about it.
Pumpkin butter is ready
It's done when the puree has cooked down to a thick and spreadable consistency. You'll also notice that the simmering bubbles are very thick and burst slowly. You can sometimes hear them sigh as they pop.
Allow the pumpkin butter to cook to room temperature. Store it in the refrigerator, in a covered container.