A facing is a second piece of fabric that supports a seam that sits inside the garment. Do you own any dresses or blouses that have a piece of fabric inside that flips up and sticks out of your collar when you put it on? that is a facing. Facings help otherwise curvy or weak seams keep their shape and look better from the outside.
Making your own facing for a skirt slit is about the easiest facing there is
You need a rectangle of fabric 2"-3" wider than your slit and length from the hem* to about 3" above the top of the slit.
Do a handkerchief hem around the outside edges.
Lay this face to the front of the dress, VERY carefully pin in place.
Sew a single line of stitches as close to your cut slit as you dare, 1/8" is preferred, 1/4" is tolerable, 1/2" is getting into pulling and stretching funky look territory.
Now you've closed up your slit with this weird rectangle hanging on the front of your dress, DON"T FRET! the hardest part is done!
Carefully cut through your facing following your existing dress slit cut (I flip it over or inside out to see this and not accidentally catch any other parts of the dress).
When you get to the top of the slit, make several tiny cuts right up to your seam in a sunburst pattern radiating out from the top point.
Turn the facing to the inside, folding it through the slit.
Iron the crap out of it. Ideally your notching at the top will create a smooth point at the top of your slit once turned. If not, flip it back inside out and trim more fabric from the top of the slit.
Hand tack the top corners of your facing to the skirt.
Whip stitch the bottom edges of the facing to the hem (if your hem is already finished, see foot note).
* ideally this step takes place before the skirt is hemmed and you finish the skirt hem and facing all in one piece. Press your hem with the facings tucked inside the rolled edge before sewing for a clean, non-shoe-grabbing closure.