There are several ways of doing this, but this is by far the easiest method I've found. If you can make your own puree, do it! Well worth the small amount of effort.
First, buy yourself a pie pumpkin. They're smaller than those that you carve for Halloween. :) Give it a good scrub.
Then cut the top off (don't take too much off!). Then cut the pumpkin in half. Using a spoon scoop out all the stringy goop and seeds (keep the seeds for roasting!). Cut the pumpkin halves into 4ths or whatever you need to do to fit it in your slow cooker. Do NOT add water or anything. Just throw in the pumpkin chunks into the slow cooker and turn it on high. Ignore for a few hours. It's done when it's tender with a fork. Let the chunks cool for a bit before peeling the skin off. Use a food processor to puree the pumpkin in batches. You can use a blender, but you'll have to add some water.
You can store your fresh pumpkin puree in the fridge for a few days or freeze in one cup batches (freezer bags work great) in the freezer for several months.