I'm not sure if you're still looking for a how-to on this, but I'll give it my best shot.
Here's the reference pic of Patty that I used, assuming you want your shorts to end up looking like this: http://www.flickr.com/photos/22513167@N04/2702081945/
Basically, to attain the poofiness would be achieved somewhat like this:
1. Take the measurement around your body of where you want the shorts to sit. Take this measurement and cut a waistband to that size. Do the same thing for the cuffs where the shorts end-- you'll want two cuffs, each one matching your right and left thigh measurements (make sure you add 5/8" in seam allomance and 1/8" in ease to the measurements before you cut your fabric).
2. Then, you'll want to take the three measurements-- the waistband, left thigh cuff and right thigh cuff-- and multiply them by three (write these three new numbers down!).
3. At this point, you'll probably want to find a basic shorts pattern (from a fabric store or pattern company's website) to base the rest off of. Now you'll want to alter the pattern so that their shorts body's waist and leghole measurements match the newer, "3x" measurements that you wrote down earlier. If you tried on the shorts at this point they'd be huge in the waist and legholes, but this is what we want. Once you have cut these pieces out of your fabric (I'd recommend using some muslin to create a mockup first), you're going to sew the side seams and crotch seams and then gather the shorts. To gather, you set your machine to the longest stitch length possible (usually somewhere around 5-- the higher the number, the longer the stitch) and sew in a straight line about 3/8" in from the edge of the waistband. Make sure while you're doing this that you DO NOT backstitch, and that you leave very long tails of string (the strings on the end of the seam that you usually cut off). Repeat this gathering process for the legholes. When you're done stitching, pull the bottom "tail" (there will be two- one for the top thread and one for the bottom bobbin thread) and the fabric will scrunch up. You're going to pull the strings, gathering the fabric until the gathered waistband and gathered legholes match the measurements of the waistband and leghole cuffs, respectively.
4. Then you'll pin, wrong sides together, the gathered seams to their respective cuffs/waistband. Sew those together (make sure you set your machine stitch length back to whatever it was before-- usually about 2.5-- and DO backstitch this time!) and turn it right side out and voila, you'll have poofy shorts!
I know this was a very long explanation but it's a relatively simple concept! If you need more help or don't understand any of it, please feel free to pm me
Good luck with your Patty cosplay!