There's very little starch in sugar pumpkins, especially the canned versions, so a mash is not always required. The pre-roasting process should be enough. But if you're using a ton of it, like Elysian Brewing Co. does for their Night Owl Pumpkin Ale, then follow their advice:
"Brewed with 150 lbs. of pumpkin in each batch. Made with Pale, Munich, and Crystal malts, green and roasted pumpkin seeds, pumpkin in the mash, boil, and fermenter. Bittered with Horizon hops. Spiced in conditioning with nutmeg, clove, cinnamon, ginger, and allspice."
I like to make a concentrated spice tea beforehand, and pitch that during secondary conditioning to taste.