The pre printed QR codes needs to be printed by the office before you can add them to assets in the field. To find out how to print QR codes see Getting started with QR codes 

Add the label with the QR code to the asset and scan it. Select if you want to add this code to a new or existing asset

Select existing asset
​Search the asset list by asset ID or customer reference

Create new asset
Fill in the details for the new asset, a customer is required to save the asset 

If a job has been created for the selected customer/asset for today, the job will opened. 

If there's no job for today the technician will be asked to create a job

Technicians that are not allowed to create jobs (setting on staff page)  get a message saying that there are no jobs for today. If you want to allow your technicians to create job open the staff page for the technician in settings and check Allow to schedule job on mobile.