Pumpkin puree is merely cooked pumpkin that's been mashed or blended into a puree.
You can peel the pumpkin, cut it in hunks, and boil it until soft. Or, to roast it, cut the pumpkin in halve (or smaller pieces if it's really big). Lay them cut-side down on a sheet pan. Add a little water, and bake at 375F until the flesh is very soft. When cool enough to handle, scrape the flesh off the rind and mash it well.
You could buy canned pumpkin puree. But that's an expensive way to go, IMO. If pumpkin isn't available, just go with butternut or other winter squash. The end result will be the same.
The difference between commercial canning and home canning is that you cannot, even with a pressure canner, reach the temperatures they do. According to USDA, the density of pumpkin butter varies so much that you cannot assure that the middle of the jars reaches the proper temperature for safe preservation.
Most people spoil garden things by over-boiling them... if they are overboiled they have neither any sweetness or beauty. Hannah Glasse 1745