You’ll need two zips the same. One, the correct length and one short one. The short one will be used for the zip pull and stop end only.
Remove the stop end as shown in the photo, and remove the zip pull. Use a pair of tweezers to lift up the prongs and pull the stop end out.
Remove the two stops at the top of the zip, the same way.
Put the spare zip pull onto the end of the zip with the flat edge facing out (round edges facing).
If you find it difficult to get the zip moving pull both ends of the zip apart gently until you feel one tooth pull apart and you’ll find the zip will move freely.
When both zip pulls are on, you can replace the stop end of your zip if you wish. If you find it easier, you can sew a piece of fabric as shown in the photo instead.
I hope this is of help.