Winter squash has harder skin than summer squash does; their flesh is firmer too and so needs to cook longer. The seeds are fully developed when the squash is ready to eat, whereas summer squash needs to be left on the vine well past the eating stage to complete the development of its seed. The seed from winter squash can easily be saved to plant next year.
Wash the seeds to remove any flesh and strings. Cure the seeds by laying them out in a single layer on a paper towel to dry. Store them this way in a place that is dry and out of direct sunlight. Once thoroughly dried, in 3 to 7 days, store them in an envelope in a cool dry place with the rest of your seed supply.
Heirloom Note Do not save the seed from squash that do not look true to their variety unless you like surprises. Squash can cross-pollinate in the garden plot quite easily, often yielding strange looking and/or strange tasting fruit.