How do you make the swirls with icing on cupcakes?I think you use the #21 but where and how do you start?
Are you talking about the swirl of icing on top of cupcakes rather than smoothing it out?
If so...
Im not sure which tip number....but use a large star tip. Start anywhere in the outer edge and make a spiral around the outer edge working your way to the center. As you move inward, move just slightly so as to make a peak in the center slightly. Good luck 
Sorry if that doesnt help
It depends on what type of swirl you're going for. Do you have a picture of any cupcake you are refereing to specifically.
I have used a 22 (gives me a spiral effect, and more of a flat surface to work with, check out strawberry shortacake and the basketball cake in my photos-the cupcakes were done with a tip 22.)http://www.cakecentral.com/modules.php?name=coppermine&file=displayimage&meta=allby&uname=leily&cat=0&pos=0
A tip 1M (Give a little bit bigger swirl) Example(made by cakefairy1
: http://www.cakecentral.com/modules.php?name=coppermine&file=displayimage&meta=search&search=1M&cat=0&pos=5&search=1M
A tip 2M (bigest swirl i have found) Example (Made by Yumfrosting): http://www.cakecentral.com/modules.php?name=coppermine&file=displayimage&meta=search&search=1M&cat=0&pos=2&search=1M
For each tip I use, I start on the outside edge and then go in a circular spiral motion and work your way to the inside (like the yellow brick road in munchkin land) Then if you want a point pull up while you are releasing pressure. If you don't want a point, stop pressure then pull up (i usually on do this if I am adding something to the center b/c I don't like the look of it -kinda leaves it like a star)
HTH
Leily
Here is a link that gives instructions:
http://www.bakedecoratecelebrate.com/techniques/1mswirl.cfm?cat=9
Tip 1 M... wilton... makes big fluffy wonderful swirls. I use it all the time
I use Wilton 1M too. I start on the outside and work my way in.
HOLY COW 
I didn't know there was a tip that big!
Quote by @%username% on %date%
%body%