At this point you should have your skirt and lining done but kept separated.
Take your zipper and pin it to the back seam of your lining (which is wrong side out), leaving enough allowance at the top to accommodate the waistband. Make sure the back seam matches the zipper teeth perfectly.
Mark where you want your zipper to end, if you use a longer zipper as I did.
Change the foot on your sewing machine with a zipper foot.
Using a basting stitch and contrast thread, sew down the zipper on one side, lift and pivot, sew perpendicularly then sew down the other side of the zipper. Sew as close to the zipper as possible
Slip the skirt over the lining, skirt being right side out. Match the back seam with zipper teeth perfectly and pin. Roll the skirt up then slowly unroll as you pin so you can see the back seam matching the zipper teeth.
Using a regular stitch, sew down the zipper as before, slightly away from the basting stitch (so you can easily remove the latter once the zipper is attached)
This is how the lining looks at this point, with the basting stitch.
And this is the skirt, with the regular stitch.
Using your seam ripper, carefully rip the seam out on both the lining and the skirt. Be careful not to rip through the perpendicular seam that marks the zipper end.
Open up your zipper and remove any bits and pieces of thread that might show and you're done!
Next step is to attach the waistband which is easy peasy!