I used the instructions
in this thread on how to install the Nokia Pro Cam. I'm not sure how easy the instructions will be for a novice but, to try and make it a bit easier, the first seven steps you perform on your computer, and only with the eighth step do you do something with your phone.
The first instruction you need to download a free program, Fiddler, that you can
download from here. After the eighth step, you can go into the Microsoft Store on your phone and, under Nokia Collection, you will see the Nokia Pro Cam, the Nokia Smart Cam, and Here Drive+. You can download all of them onto your phone for free.
Basically, what you are doing is routing the WiFi of your phone through a proxy server on your computer, and the proxy is telling Microsoft that you have a Nokia Lumia 1020 instead of a 520.