Put on the pants and mark the length of the skirt you want, using the dressmaker's chalk and string. Wrap the string around your pant legs an inch lower than you want the skirt to leave room for a hem. Trace a line on the pants where the string touches it.
Remove the inner seams on the pant legs, inserting the point of the seam ripper between the two joined pieces of cloth and sliding the curved blade to cut through the thread. Remove the strands of cut thread with your fingers.
Trim the pant legs at the chalk lines you just marked.
Lay the pants flat on the floor or a flat work surface. Arrange the legs so that they lie straight down from the sides of the waist area, not splayed out.
Pin and sew together the front of the legs. Make the seam at the point where the fabric of the pants naturally overlaps after you arranged the legs to lie straight. Turn the pants inside-out to sew them. Use a small running stitch, or straight stitch, on the sewing machine.
Try on the pants again to measure the piece you need to fill the gap in back. Stretch the dressmaker's tape measure over the bottom of the triangular gap that forms in the back to see how wide this gap is.
Cut an isoceles triangle of material to fill the triangular gap in the back of the pants. Make this piece as wide as the measurement you just took, plus a 2-inch margin. Make it as tall as the slit up the back, plus another 2-inch margin.
Pin and sew the triangular piece into the back of the pants. Line up the edges of the slit in the back with the tall sides of the triangle piece you just cut. Again, use a running stitch and sew the garment while it's inside-out, creating a hem on the inside of the skirt.
Fold up the bottom inch of the skirt to the inside of the skirt. Iron the fold in place. Sew the folded hem in place with a running stitch on the sewing machine.