Simple answer:
Download the prop map by searching for the sofa on
Wraith's Site.
All you do is right click the image and save it to your PC.
Then open it in Gimp or Photoshop or whatever image editing tool you wish to use and make your changes
Here is the key:
You have to upload your completed image to the cloud. Most use Photobucket but any image hosting service will work that supports redirected links.
Once you have your image uploaded simply use the "attach cached web image" option on the sofa and paste in the direct link to the hosted image and whallah!
The most common mistake is assuming you can upload your completed image to RLC. RLC does not host modified propmaps for us.