The office is located near Quito’s Mariscal district and is called the Dirección General de Extranjería (now located on Avenida 6 de Diciembre between Colón and La Niña). You’ll need to bring along your passport (plus a colour copy of the photo page, and another of the visa stamp and entry stamp pages) as well as passport photos and some cash to pay for the process (around $4). You will also be asked to provide a folder with metal clasps (carpeta con brinches) which you can buy at any papelería. You will also need proof that your visa was approved by way of the Certificado de Visación. This should have your photo in the top corner.
The office is open Monday to Friday, from 8.30 AM until 4.30 PM (though this is always subject to change). It’s a good idea to arrive early, as the office can get very busy.