What you need :
or needle ( will take forever)
strong jean thread
pattern paper (or a replacement)
seam ripper (optional)
pencil or pen
1.Start off with a pair of overalls (to make a dress) or a pair of jeans (to make a skirt). Make sure the the waist of the jeans still fit you so when you're finished you can wear your new skirt. Same with the overalls. (If the overalls are short, this doesn't matter because it is going to be a dress and the dress will end up longer than the pant legs fit you currently)
2.take a seam ripper or a pair of scissors (more difficult) and undo the inseam (this is the stitching between your legs when you wear your pants). The top picture shows you what the inseam looks like from the outside. The second one shows you what the inseam looks like in the inside (the blue thread stitching in the second picture). If you want the bottom of the skirt to unravel some, to have a frayed look then undo the hem at the bottom of the pants (the hem in the second picture is the vertical seam in the picture with the cream colored thread (horizontal on your pants)). If you don't want the bottom of the skirt to unravel then just undo hem like 3 inches on either side of the inseam. The below picture shows you how far you undo the inseam in the front of the pants (up to the zipper, not past the reinforced stitching below the zipper). (undo the back where the two pant legs are attached the same amount as you did in the front (3.5 to 4 inches).
3.The above picture shows you what your pants will look like after you've finished undoing the inseam.
4. Lay out the pant legs so that they lay out straight naturally. Pin the the fabric that over laps (where the pant legs are attached together) Try on the pants trying not to hurt yourself or make sure they fall right. You can baste stitch them (sew with the largest stitch on your machine or a very large hand stitch, it's large because it isn't supposed to last, it's temporary) to try them on.
5.Take some pattern paper (you can find it at your local sewing store) or tissue paper (the kid for presents) or craft paper, etc ( a paper on a roll or large sheets so that it is big enough for this). Put the paper between the pant legs. Then trace between the pant legs (you'll need to do one for the front and the back). Shown above.
6.then for walking room add some length on either side (I added like 9in). This is so that you can walk in your skirt when your finished.
7.Cut out the pattern (the traced inseam with added walking room) shown above.
8.Place the pattern over your fabric and cut the fabric out in the shape of the pattern. It may help you to pin the pattern to the fabric so that it doesn't move.
9.The inseam has this automatic fold on one side (iron the other side so that it has the same fold inside).
10.Place the pants with the front on the floor. Place the fabric you cut out for the front (place it right side down on the floor, the side you want to show) on top of the front part of the pants (in other ward inside the pants you can open as much as you can the pant legs so it is like the pants are one layer of fabric and not two).
11.Fold the fabric sides under the fold in the inseam and pin. When you finished pining the sides tuck the top point under some (make sure it isn't going to stick out the other side). When your finished pining the fabric it will look like the picture below. Do the same for the back (it may be easier to sew the front before you pin the back)
12.When you sew it start with the pants turned inside out. Just do a regular straight stitch starting at the bottom of the zipper and going all the way down to the bottom about a centimeter away from the edge. (the white stitching in the picture below is what you sew first)
13.Then turn the pant right side out (like normal) and sew as close to the edge as possible.
14.Then using the zigzag stitch. Sew over the inseam that didn't get sewn on the inside (the part over lapping under the zipper). Continue the zigzag over the edge of the fabric you used to fill in the pants to make it a skirt. details shown in above and below picture.
15. If you want the skirt to fray some at the bottom then sew two straight lines across the bottom An inch from the bottom and about half an inch from each other. All the way around the bottom of the skirt.
16. If you don't want the skirt to fray then fold the skirt under twice about an inch each time and sew around the inside folded edge all the way around.
a couple more pictures here:http://www.teengirlclothing.com/jean-denim-skirt-diy.html