Preparation. Your mirror likely has an attachment plate at the back. Examine this plate and hold it against the windshield to determine how to align your mirror. Many windshields feature black spaces indicating the mirror location; this is where you need to attach the mirror. If this is not the case, use masking tape to mark the proper location on the outside of the windshield.
Remove attachment plate. This makes it much easier to re-attach the mirror.
Prepare both surfaces. Remove any old car glue from the attachment plate and the windshield. Your surfaces must be clean and dry for effective bonding. If your car adhesive needs an activator, apply this to the windshield and/or the attachment plate. Also make sure the temperature of both parts is appropriate for your adhesive.
Apply car glue and position plate. Apply the automotive adhesive to the center of the attachment plate side facing the windshield, and hold it against the windshield for the length of time indicated in the product instructions. You need to position it correctly the first time because trying to reposition parts can damage the plate or your windshield.
Let car glue dry. The car adhesive needs to cure before you can affix your mirror to the attachment. See the manufacturer’s instructions.
Attach mirror. Slide it into place on the attachment plate. Secure using screws, if needed.
Clean-up. Cut away extra cured adhesive using a sharp blade, or remove it using acetone or boiling water. You can also apply water to the adhesive, let the latter cure, and then scrape it up.