This is an alteration, not a repair! Charge lots! It will often take a lot of time
With horse riding boots, where you decide to run the new zip down the back seam, sometimes there is a piece of leather over the back seam, cut this off about 25mm (as low down as you will be able to restitch –try the boot on the stitcher) up from the bottom, it will cover up the bottom of the zip.
Remove the rest of the piece of leather that runs down the back.
Insert the new zip between the upper and lining.
Take 2
This is the right side of the left boot
This picture is only if you are trying to widen the boots as you put the zips in!!!!!!!!!!