Either you can pipe your stitching using a #1 or #2 tip if you want a really prominent look, or you can use a stitching tool for a more subtle look. You can get a stitching tool at Michael's or Joann's.
HTH! 
Hi! Hope I can help. When I want to do stitching, I use a sewing tool. I'm sure there's a name for it but I can't remember. It looks like a miniature pizza cutter with zig zags on the wheel. When you roll it right inside the edge of the gumpaste or fondant it will give you tiny holes.
Good luck!
As was mentioned, you'll need some sort of stitching tool such as this one: http://www.globalsugarart.com/product.php?id=17413
OR, you can use a toothpick and white coloring, like this: http://www.flickr.com/photos/natiscakes/3184076630/
(click to enlarge both so you can get a clearer view)
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