To turn glass into a mirror at home you can use a specialized mirror effect spray paint.
You apply the mirror effect spray paint to the backside of the glass and it essentially coats the glass with a reflective layer which is commonly called looking glass.
Before applying the mirror effect spray paint to the glass be sure to clean the glass completely and dry it before applying the spray paint.
To apply the mirror effect spray paint hold the can of spray paint a consistent distance from the glass and evenly coat the entire back surface of the glass.
Then allow the paint to dry.
Most of the commercially made mirrors are made out of plate glass with a reflective coating (aluminum or silver) that is then applied to the back side of the glass.
Front surface mirrors are made of plate glass with a reflective coating that is applied to the front side.
Telescope mirrors are made out of pyrex or other thermally stable types of glass.
Mirrors are also often produced by the wet deposition of silver, or sometimes nickel or chromium (the latter used most often in automotive mirrors) via electroplating directly onto the glass substrate.
Glass mirrors that are used for optical instruments are usually produced by vacuum deposition methods.