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You have to make sure you don't have too many sts for the dbl pointed needles or they'll fall off the ends. You can pick up the sts with a dpn, then slip them onto the circular. You don't have to decrease the sts, you can have just a straight bottom instead of a curved one, but you need to close up the bottom some way. You can leave the sts on the needle and do a three needle bindoff, or kitchener the sts together, or bind them off and sew it closed. There are videos for all these things under Advanced Techniques, Tips and Cast off.