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 ?
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.
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
Install a zipper foot on your sewing machine to allow stitching close to the teeth.