You may want to go a little bold or want to add some room to the skirt hem – whatever be the reason, you can quite easily add that slit on the seam of your skirt.
Here is the tutorial – I assume that the skirt is already made and you want to add the slit now. If you are making it from scratch you can add the extra for the facing of the slit as part of the pattern itself.
Decide on the length of the slit you want.
Take out the stitches at the hem of the skirt, and Loosen the stitching on the side seam to this length on the side seam of your skirt plus about 3 inches up.
You need to trim the seam allowance of the side seam to 1/2 inches.
Cut out 2 pieces of fabric 2 inches wide and length equalling the length of the slit (plus the hem allowance) – this is your facing for the slit.
Keep the skirt right side out.
Keep the facing piece on the fabric edge (of the slit) and sew with a 1/2 inch seam allowance. Do this for the two edges of the skirt – back and front.
Now sew the seam – till 1 inch past the facing.
When you open this now, and fold the facing to the back, it will look like this.
Now told the facing edge once and press in place.
You can hand sew the edge to the outer fabric or sew with the machine. Sewing by hand is preferred.
Another method to sew a slit is to make a Kick pleat – in this, the slit is not visible at a glance. It looks like an inverted pleat. You can learn how to sew a kick pleat here.