Remove the old zipper by unpicking the stitches with a seam ripper
Open the seam enough to fully free the zipper
Measure the old zipper and get a replacement of the same length
Match the zipper type and color to the original
Position the new zipper in the opening
Pin or clip the zipper tape in place
Baste the zipper if needed to hold it steady
Sew one side of the zipper tape to the fabric
Close the zipper and align the other side
Sew the second side of the zipper tape
Check that the zipper opens and closes smoothly
Reinforce the top and bottom ends with extra stitching
Trim loose threads
Press the seam flat if appropriate