UPDATE: The first, FIRST thing you have to do is make the chair.

The first thing is to cut your fabric. I made two chair slipcovers, and because I used directional striped fabric, I layed out the pieces as shown in the diagram. If your pieces do not have a direction, you could lay them out differently. Also, I made my skirts 12" long - you can make your skirts any length with 18" being floor length. From 45" wide fabric, you will need to cut two strips per chair.

Take the main seat/back piece and fold it over with right sides together so the fold overlaps 16 1/4". Pin and stitch with 1/4" inseams along edge.

At this point, I recommend fitting the slipcover over your chair to make sure the fit is right. If you are satisfied, remove and then clip in 1" at base of fold over as shown in diagram.

Then fit slipcover back on chair, still with wrong side facing outward. The corners will come to a point as shown in picture. Pin and draw a line over the excess fabric as shown in photograph. Remove and stitch over line. Clip off excess fabric leaving 1/4" for seam allowance.

This completes the seat/back! Turn so right sides are out and fit over chair. Now all we need is the skirt!

For the skirt you will need a long strip. Sew the two strips together to create this long strip using 1/4" inseams as shown in diagram.

Now we just got this back piece of the skirt to figure out. No biggie. What I did was sew the two skirt ends together first. Then I found the center of the back piece and matched up with center back seam of skirt and pinned. Then I pinned excess fabric into a kick pleat with right sides together. And then I just stitched it all up. Super easy.

And then I just hemmed the entire skirt by pressing under and stitching. You could even hem before attaching skirt to seat/back but I wanted an opportunity to adjust the hem if necessary.

And that's it!