You don't have to slip the stitches, just use one of the dpns to knit with instead of the other end of the circ. Knit about 1/3 of the stitches on it, then take the 2nd dpn and do the same, etc. Continue in pattern doing the decreases as you have been. At some point, when you reach 9-12 stitches the pattern should say something like `cut your yarn and thread them through the remaining stitches'. You do that and pull the yarn end (leave about an 8" tail) tight and it acts like a drawstring to close up the top of the hat.