Take your skirt, lay it out on the floor. Measure from your waist to your crotch, and add 5 inches for extra room. Then what you do is you measure from the waistband on the skirt to however long you measured for your crotch of the pants, but lay this measurement down the centre front of the skirt.
Cut from the bottom of the skirt up to that mark. Before you cut, make sure all seams match. Its hard cause its crinkle skirts.
Then, sew up the seam as you would the inside seam of a pair of pants.
Then, make elastic casings on the bottom of the pant/skirt. Leave yourself about 5/8′ of room to fit in an elastic.
Run the elastic through, sew in the elastic, and you’re done.