How to make pumpkin purée
There are lots of ways to do this, but this is here I do it.
First, take your pumpkin and cut it in half. Scrape out all the seeds. Place both sides, inside up, on a sturdy, rimmed baking sheet. Pop it in a 350° oven until it is nice and soft. The amount of time it needs will depend on the size and type of the pumpkin, but plan for about 90 minutes to two hours.
If you can't cut the pumpkin in half, or don't want to, it is acceptable to stab it all over and bake it whole. It will be much, much easier to open up after it has baked, although removing the seeds gets a little more complicated.
By the way, if you took out those seeds before baking, save them to roast!
After your pumpkin has baked awhile and is nice and soft (the softer the better, but don't let it burn), take it out of the oven and drain out the extra liquid that has probably collected inside. Scoop all the flesh into a food mill and get to work. This will take awhile unless you have a fancier food mill than mine (like this one). It was hard work to get the end of the pumpkin through, but I persevered because the nice thick stuff comes through at the end.
Stir it all up and bag. I put two-cup portions into quart freezer bags, because that's about how much is in a can. It's a lot of work, but it certainly saves time later when you need some pumpkin purée and all you have to do is pop a bag in the microwave for a few minutes.
I got 18 cups of pumpkin from my two medium-sized pie pumpkins.
P.S. After you're done roasting the pumpkins, you can toss your clean, dry seeds with a little olive oil and some salt and roast them at 350° until they start to turn brown, about 20 minutes.
P.P.S. After your pumpkin seeds are done roasting, you could certainly throw a pie in the oven while it's still hot, or you could put it in with the seeds if you have it ready in time.