With the rod welded to the patch, you now have a "handle" on your patch. Have an assistant hold it in place, if you gas weld or you can work both by yourself with a mig. There are some people that can gas weld while holding the patch, but I doubt they would be reading this. :-)
The pic shows just a piece of metal with patch in place. This can be related to a firewall, where you DON'T have access to the back, or an obscure lower panel.
Sorry about the lousy picture quality. It was hot, and I needed a beer!
If you have a hole in the firewall that needs a patch (To smooth the firewall), take a piece of tape, larger than the hole and apply it over the hole. Take a dirty finger and trace the outline of the hole on the tape. Remove the tape and place it on a piece of patch metal. Cut the metal to your template. Take a piece of welding rod, and weld it to the center of the patch. Trim as required to fit the hole as snug as possible.
A no brainer, but block sanding your bondo is required prior to paint. IF you MUST use your fingers, and you CAN'T use a block, fold up you paper into a small block, you that your fingers don't apply the pressure. Otherwise, you will have grooves where your fingers put more pressure on the paper.
One of the things that gives Bondo its bad name is bubbles. Even with careful folding/mixing, you will get small air bubbles. It is imperative that they be removed. Scratch around and be sure they are gone, or they will rear their ugly head when your paint is finished. Take the time to get them all!
Here, Gene made mention of the correct way to use a cheesegrater. Going diagonal will prevent you from leaving "tunnels" or grooves. Grate at 90 degrees from previous pass to help level it off. You should begin grating when the filler is the consistancy of cheese.
(This series is for the benefit of newbies. Most of us know this material, but it was presented for those who haven't gotten to play with Bondo. (Yes, there are such people..even at Gene's seminar !!)