Okay, at this point in the process you have the front skirt, with quilted lines going on the bias in one direction. I marked your original line with blue painters tape, and used a quilting guide on a walking foot to keep the rest of the lines even.
Use the blue tape to stitch your first line, and then your quilting foot guide to make the rest of the lines. You'll have a nice and neat grid.
Now to quilt the back pieces. Use the same method of marking the first lines with blue tape. Start the tape in the upper corners like you did with the skirt front, and have the lines go at a 45 degree angle. Make sure that they will meet at the same point in the center back, in a V shape.
Quilt the two back pieces in both directions. Now all three pieces are quilted!
Now it's time to sew the side seams. Place the back pieces right sides together with the front piece. Match the quilting lines as best you can by using a pin as a guide. Poke the pin through the fabric, and then adjust the two layers of fabric until the pin is going through the two lines of stitching that you are matching up.
Next, sew the center back seam, but only up to the point where the zipper will be inserted.
Press all seam allowances open.
I chose Hong Kong seams for my skirt, and used the contrast quilting fabric. Finish all seams, including the back zipper opening.
Insert a zipper at center back. I used my favorite method, the lapped zipper.
Next, I cut the waistband, leaving seam allowances and 1" for an underlap. The waistband is 1.5" wide when finished.
I used a nonwoven medium weight interfacing on my waistband. To give a slightly different look, I used 1/4" channel stitching on the waistband. This provides both stability and visual interest.
I sewed on the waistband, leaving the 1" underlap at the right back. The waistband closes with a hook and eye.
I turned up a narrow hem, and finished the raw edge. Hem lace is perfect for this purpose.
And that's it! You have a cozy, perfectly '50s-style quilted skirt!