By skyforbes 
Attaching Your Facing to an Invisible Zipper.
and back facings to waistline, matching up
notches.
Fold back side seam edge of facing (at zipper
tape end) 3⁄4″ or 2cm towards wrong side of facing.
Pin in place on zipper tape.
Next, reach in underneath facing, holding facing seam allowance to garment and zipper seam allowance.
Remove top pin. Flip facing to wrong side and pin 3⁄4″ or 2cm on wrong side of facing along garment seam and zipper tape allowance. Note: Check that facing overhang distance from seam allowance edge is the same.
Move needle position to the far left and stitch facing in place to garment seam allowance. (Note: Your machine foot should be butted up to zipper teeth)
Note: check that you have lined up facing waistline to garment waistline edge. Repeat on other side of zipper opening/facing with needle to the far right.
Cut off exposed tops of zipper tape and excess seam allowance on facing edges.
Next, with front and back facings turned towards the wrong side of garment you should have the lower edge of the facings lined up evenly.
To finish off the waistline, open up zipper and pin right sides of garment and facings together. Pin in place matching up centre front and back, dart notches and closed side seam.
Make sure to turn seam allowance towards facing side and pin in place.
Sew the facing to the waistline taking the correct waist seam allowance, starting at one zipper opening and finishing at the other side of opening.
Tear away Vilene shields at waistline edge ( if you’ve use vilene shields on the waistline). Turn facing out away from garment and understitch on the facing, making sure to catch the seam allowance at the back of stitching.
Turn facing and zipper in towards the wrong side of skirt. You’re done!