Oh, it's easy.

Fill a nalgene bottle. Pour contents into the bag. Mark it. Repeat. Twist it closed as low as you can to the water. Use a pair of scissors to cut the extra length off, leaving enough to get a handhold on. Empty it and allow it to dry. Roll it up.

In my PSK I still have a first generation oven bag, the un-metalized mylar kind. In the other kits, I carry the current ones, I don't like them as much, bulkier.