a petal nozzle looks like this shape
cover the cupcake with some butter cream and make a small mound in the centre
then holding the nozzle sideways with the wider part of the nozzle nearer the cake start piping, its easier to turn the cake rather than the bag for the centre part of the rose.
with an even steady squeeze pull the butter cream in a spiral shape around the mound.
then with the bag in the same position, pipe small petal shape in a semi circle shape over lapping each one, do this by pulling the bag up in s circle shape flicking your wrist. continue adding some smaller petals then add some wider ones the near to get to the edge, then make some small ones to fill in the gaps. it looks allot harder than it is it was really easy.
this video on youtube is really helpful, watch it here