These white jeans from Ross are the ones in question. As you can see, they look okay without alterations, but when you see the before and after at the end, you'll realize what a difference the alteration makes.
Step 1. (make sure your jeans are pre-washed/shrunk so that no shrinking occurs after you've altered them) Try on your jeans, inside out, and mark with a pin the starting and stopping location on the inside of the leg. My starting point was right around my knee, and the stopping location was about an inch from the edge at the hem of my pants (as seen below).
Step 2. Remove the pants and, with a straight stitch (and sturdy sewing machine needle, if you're working with thick denim), sew a slightly angled line from your starting point to your stopping point. Make sure your starting point is exactly lined up with the original seam, and then gradually angles away from it, down toward your hem.
Turn the pants right side out and try them on (as seen below). If the skinnifying is to your liking, repeat on the second leg. If not, make any changes and then move on.
Step 3. Once both legs are altered, cut off the extra fabric and zigzag or serge your raw edge. And you're done!
Check out the difference that taking in one inch can make:
And here's a little closeup so you can really see the difference:
As always, comment or email with any questions! Happy Sewing!