The appointment is booked on the UKVCAS website, usually done before you upload your documents, though the documents can be uploaded at any time between registering on the site and about 2 days before the appointment.
You book an appointment here:
https://www.ukvcas.co.uk/appointment-infoThere are various types of appointments you can book... free appointments are very hard to find, so it's likely you will have to pay for an appointment, which can cost up to about £130. There are appointments at Enhanced centres for about £70, but they already include document checking and scanning.
Document scanning is an extra service that can be added to an appointment for about £45-50 (if it is not already included), but it is not something you can buy without booking an appointment... I believe you select an appointment first and then you can add document scanning to it if you want to.
So, the process usually is:
- register with UKVCAS
- book biometrics appointment and add any required services (like document checking and/or scanning)
- upload your documents before you attend the appointment