The difference between boot cut jeans and skinny jeans rests in the fit between the knees and the ankles, with skinny jeans fitting tightly around the lower leg and boot cut jeans being a looser cut. Altering your jeans to be skinny jeans is simple and quick.
Turn your jeans inside out and put them on.
Stand in front of a mirror. Start at the knee area and pinch the outer seam of the leg, securing it with straight pins. Pinch and pin the denim all the way down to the ankle. Leave enough room at the bottom so that you can still slip your ankle and foot out of the jeans. Repeat this step on the other leg.
Take off the jeans and lay them out flat on a work area with the pins facing up. Draw a new stitching line with the fabric pen to show where the new seam will be, using the pins as your guide. Make the line extend from the bottom hem up to about 2.5 to 5 cm (1 to 2 inches) above the knee of the jeans. The new stitching line should taper to match up with the original seam at the knee area.
Move the straight pins over about 1.3 cm (1/2 inch) from the new drawn seam line, smoothing any bunched material in the process. Moving the pins allows room for the metal sewing machine foot as you stitch.
Stitch the new seam in each leg along the drawn lines. Remove the pins.
Cut off the excess denim, leaving approximately a 1.3 cm (1/2 inch) seam allowance, to prevent fraying. Turn the jeans back to right side out. Iron the new seam.
Hem the jeans, if desired. Put the jeans on. Mark the desired length on each leg with the fabric pen or with a pin. Remove the jeans, lay them on a flat surface and cut off the excess, but leave about 1.3 cm (1/2 inch) extra edge for the hem. Turn up the edge, pin and stitch the hem.