Scott Clark Woolley's book has instructions for them as does Sylvia Weinstock's book. Hope that helps some!
Buttercream or Gumpaste? Cupcake Cafe does some lovely Iris in buttercream. From looking at the picture I would say they used a #125 rose tip wide end down, piped an elongated petal by starting at the base, moving your hand up till the petal is the size you want, move your hand a bit to the right and bring back to the base, do this all in one smooth motion. Then pipe a second petal beside it but with a tiny space between the two petals. Here is a photo of Cupcake Cafe's Iris cake.
http://www.cupcakecafe.com/cakes/wedding/w_1.htm
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