This is what I did:
Take one pair of jeans
Figure out how long you wan your new skirt to be and cut off the pant legs, adding an additional inch (for finishing the hem).
Open the inner leg seams, both sides to the crotch and then back and front to the base of the zipper and to where the back no longer curves for your derrière.
Turn your jeans/almost skirt inside out and fold in the open edges and press into a "V", front and back.
Turn right sides out again and lay on a flat surface. Place pins to hold pressed seams.
Open the same seam (inside leg) on the pieces of pants leg that you cut off. Press open.
Place a cutting mat, or cutting board in the area of the "V" of the skirt and lay the (now) rectangle leg piece into the opening. Using the cutting mat as the pin stop, pin the skirt "V" opening to the pant leg material filling the opening. Pay special attention to the area around the zipper making sure the additional material is tucked neatly under.
Stitch a double row of stitches along each side of the "V".
Turn inside out and trim excess material leaving 1/2 of denim to the side of each seam.
Repeat on the back.
Trim hem line to an even edge and her you make a choice. You can either turn under the hem and again, double stitch, or you can leave the hem be and it will fray softly with repeated washings.
Enjoy your new skirt. {If you are totally confused, go check these tutorials here and here}.