I always make my facing pattern so that it matches the top of the skirt pattern - check to see whether your pattern is the same, if not you will have to adjust accordingly.
Now sew the sides of your facing to the zip edge a foot width away from the zip coil. Because my pattern pieces are the same, I need to overlap the facing by 1cm. You can trim this overlap off later if you prefer, but I usually don't.
Repeat on the other side. I always stitch from the top down, so on one side the facing is on the bottom, and on the other side the facing is on the top.
Open out your facing and it will look like this:
Now for the top edge - fold the skirt panel around the coil of the zip onto the facing, so that the coil of the zip is right in the foldline.
From the top your arrangement should look like this:
Begin about 3mm above the end of your zip coil, and stitch your facing to the skirt, matching all notches and side seams, to the other end. Make sure you finish 3mm above the end of the zip coil at this end too, so your top edge is even when zipped up.
Turn through to right side. To get a nice square corner, it helps to turn the corner of the seam allowance towards the zip and pinch it as you turn it through.
But you haven't finished yet!
Because the top edge of your skirt is curved in places, you need to tape it to prevent it stretching. I use a 6mm wide polyester twill tape for this, and preshrink it under heaps of steam from the iron. Notice how one edge is straight and the other is wavy?
The straight edge will be placed uppermost in the finished skirt, so the slightly longer wavy edge flares to match your shape. Trust me, the tape may only be 6mm wide, but inserting it the right way up makes a big difference!
Measure the tape to fit your pattern exactly (excluding seam allowances and darts), and mark it at the side seams, CF and CB, and darts. The measurement should be done along the stitching line, not the cutting line.
Then sew the tape to the seam allowance in the top edge of your skirt, matching all your markings. I stitch closely to the straight edge of the tape, which is positioned 1mm away from the stitching line. Once the facing is turned, the wavy edge of tape will be lowermost.
I usually trim 1cm off the beginning and end of the tape to reduce bulk in the corners.