How do you create/edit a camera overlay?

Hi there

Had a quick search but did not get any hits for this....

I have a reticle I'd like to put on a camera, but I have no idea where to start. I'm not a programmer and don't know how to code.

Camera is this one -
https://steamcommunity.com/sharedfiles/filedetails/?id=707074528

Reticle is here. You can see the yellow part is mine, placed over the top of the existing reticle. The markings from the bottom up are ranged at 2KM, 1KM and 500M (when zoomed in fully). -


The ranging is based on this weapon -
https://steamcommunity.com/sharedfiles/filedetails/?id=1745194693

Thanks in advance!