My credentials: I literally grew up in a fab shop that fabricated brush guards, headache racks, skid plates, custom beds etc for oil exploration companys.
For a brushguard we would cut two plates to mount to the frame that extended forward. These were called dog ears. No need to weld on the frame. Bolt the dog ears to the frame and weld onto the dog ears.
Find some mounting holes on your frame and go from there. Drill holes only if you have to.
The only time we welded on the frame was to either extend or shorten the frame itself and there were special processes to follow.
Yes, disconnect BOTH battery terminals and have your ground as close to your work as you can get it. Clamp the ground on your dog ear, not the frame
Now go have fun :)