Replace a small zip in jeans/trousers using the same method for any jean/trouser, however the challenge comes in finding a zip the exact size.  If the jeans are hipsters, a zip will have to be cut down to size.  The best way to do this is to use bias binding or fabric across the bottom of the zip.

Some people cut across the top.  I do not agree with this theory on the basis that without the stopper at the top, a slider can fly straight off if too much force is used.  The knobs at the top of the zipper are there to stop the zip flying off.  Why not leave them there and shorten from the bottom.

Determine the length you want the zip to be.  Cut a piece of fabric about 3 cm (1 1/2 in) square or cut some bias binding the width of the zip.  Lay it just above the length you want the zip.  It does not really matter whether you sew it on the right side or the back side.  Stitch the fabric onto the zip with the excess fabric on the left.  Step over the metal teeth with care.  Cut the excess zip off using normal house hold scissors.  Don't use your good fabric scissors or you could blunt them.  Iron the fabric flat, folding in the sides and turning it to the back.  Stitch the folded section down so that the fabric or bias is wrapped around the bottom of the zip.  This is a very secure method for shortening a zip.  You are now ready to replace your small zip.

Happy sewing

Judith aka genie