after practicing on an old pair of jeans, I learned some little tricks and then was brave enough to narrow the legs of a brand new pair of jeans. here's how I did it...

you will need:
jeans, pins, sharpie, yardstick (or meter stick for the rest of the world :-), scizzors, sewing machine.

first- put the jeans on and mark with straight pins the point where you want to start narrowing (most likely right at your knees)
http://www.flickr.com/photos/4.....493548645/

next, turn the jeans inside out, lay flat on a table, front side up, and remark those same spots
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If you can afford to lose an inch in the length, cut the current hem off. if not, you will need to rip out the hem stitching and take down the hem (iron it smooth).
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Note that when your jeans are laying flat, the seams aren't on the sides, but are in a bit from where you have them folded. this is because the side seams of jeans are sewn slightly forward to create a more slender look. The back width of the jeans leg is a bit wider than the front width. We are going to keep this proportion with out newly narrowed jeans.
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now, you need to determine the finished width that you want at the hem of your new pants. I measured my current favorite pair and noted that the width at the hem is 5 3/4 inches (note, this is the width with the jeans laying flat, not the circumference)
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measure the current width of the pair you are altering in the same way. I measured mine at 8 3/4 inches.
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subtract the width you are wanting from the width you currently have and divide by 2. In my case.... 8 3/4 minus 5 3/4 = 3 inches. Divide that number by 2.... 3 inches divided by 2 is 1 1/2 inches. This final number is how far you are going to measure in from both seams at the hemline. Mark (in my case 1 1/2 inches) in from both side seams.
http://www.flickr.com/photos/4.....494432714/
http://www.flickr.com/photos/4.....otostream/
http://www.flickr.com/photos/4.....otostream/

Now, use your yard stick to draw a straight line from the straight pin at the knee, to the mark at the hem. Yur line will begin AT THE SEAM of your knee point.
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your jeans should now look like this.
http://www.flickr.com/photos/4.....493556697/

Before you sew along the lines you drew, note that you will be readjusting the seams at the side. You will fold along the current seams before you begin sewiing. this will allow your finished jeans to maintain the proportion of the front panel being narrower than the back. I hope this makes sense to you.
http://www.flickr.com/photos/4.....494327352/
http://www.flickr.com/photos/4.....otostream/
http://www.flickr.com/photos/4.....otostream/

Now you are ready to sew along the lines you drew. trim the edges (I use pinking shears) and re- hem the bottom of your jeans.

Please feel free to ask if you have any questions. I hope my explanation has been clear. Have fun and happy sewing!