Make life easy on yourself and insert your zip as early as possible in your garment assembly. It really is much easier to sew a CB zip into just your two back panels, than to try and sew it into a nearly completed skirt!
Here is my skirt ready for zip insertion - as you can see I have already sewn my side seam beneath the zip:
If your fabric is delicate the base of the zip around your backtack should be fusetaped for reinforcement. For this viscose georgette you will see I have fusetaped the complete zip seam, and I still need to overlock the yoke seam before I insert the zip.
Open your zip and place it on your zip seam so right sides are together - ie, your fabric is face up and your zip is face down. I always use 1cm seam allowances, and as the zip tape is the same width as my seam allowance I just need to line up the edges. The pinked edge of the top of the zip should align with the top cut edge.
Notice how I use an ordinary zip foot? I filed my invisible zip foot in the rubbish bin years ago, as I get much better results with an ordinary one. I can stitch closer to the zip coil - just fold back the coil and stitch in the groove - so you never see the zip from the outside. I can also stitch closer to the end of the zip - no more gaps!
When you near the bottom lay your seam allowances open and flat, and fold back the other side so you can locate the position of your backtack clearly. Butt the zip coil adjacent to the backtack and stitch right down to this level.
Now you're halfway there!
Now see how my skirt has a trim along the yoke seam? To make sure both sides match exactly when the zip is closed, I clip the zip tape exactly level with this seam.
Then I close the zip, and clip the other side exactly opposite my first clip. Now when you sew the other side of your zip, ensure that you position your yoke seam at this clip, and voila - they will line up!
It doesn't look very exact in the photo does it? But it is!
I also clip the base of the zip tapes in the same manner, exactly opposite the end of the seam. This is a trick I learnt from my sample machinist, and I found it really helps keep the zip tapes parallel.
Now you'll need to open your zip again to sew the other side. With your seam allowance open and your zip coils butted together, stitch the from the bottom up, making sure any notches align, and that the upper zip aligns with the upper edge, by the same amount on both sides.
Now you can slip the zip puller through the little gap at the base of your zip - you can just see it peeking through here - and zip it up! Check everything aligns - top edge, yoke seam, no bubbles at base of zip - and you're done!
Now who said invisible zips were tricky?!
Look out for the next step soon - all about finishing the top edge and facing your invisible zip, neatly and by machine...