Replacing a zip is either an awarding experience or makes you want to give up sewing all together. I recently replaced a zip for a customer, here are some tips that helped me do a good job without tearing your hair out.
I know this sounds silly but before you buy a zip, take note of the type of zip that is on the skirt.
Also measure it on the inside (so you can see all the zip) so you get the size you need. If you get a weird number like 17cm round up to 20cm, it's more likely the zip was cut down.
Also if buying take the garment/item with you, just to help with getting the right colour, or the one closest to it.
Sewing the zip in using a tack stitch by hand helps keep the zip in place while sewing. Don't just pin, normally just pin is fine or sewing free hand, but I find pins are more likely to cause the zip and fabric to move more.
I hope that helps and if you have any other questions just ask.