For visual learners, this video from Lauren's Top Tops shows the standard centered zip method, while this video from Professor Pincushion covers the process of installing a zipper in a seam. To make sure this fits your project, are you: (clothing) or a bag ?

Use a top-stitch for a clean, secure, and professional look.

Pin the fabric pieces right-sides together and sew from the bottom up to the marked point where the zipper opening begins using a regular stitch.

Installing a zipper, or a separated (like a jacket) one? Let me know, and I can tailor the instructions further. Install a Zipper in the Center of a Seam

Open the zipper. With the garment wrong-side up, place the upside-down zipper (head facing down) over the seam allowance, aligning the teeth perfectly with the seam line.

Once secure, carefully remove the basting stitches in the middle of the seam to reveal the zipper. Finishing Touches

Turn the fabric over to the right side. Start at the bottom, stitching up one side of the zipper, turning at the top, and coming back down on the other side.