Hello!  My friend picked this cake for a baby shower and I have no idea how to go about getting the rainbow to spiral up the cake!  It looks to be one continuous rainbow.

  Any suggestions??

Use a wide ribbon to "map" where it should go, then mark with an edible marker. Then slide it over, and draw again, parallel to the first one. The wide ribbon will help to insure you don't have overlap, because that would look bad. You will have to use a lot of common sense.