It should be done at a body shop. When I did my 92 Civic, I just drilled the spot welds to remove the support and used self tapping screws to reattach the new support. I had to have the car back on the road after the weekend though.
It did last quite a few miles and years with no problems. If you live in a rust belt, paint the support first. I didn't and it was pretty rusted through on the bottom when I finally parted the car.

If you live close to a shop you could drive the car over and have them tack the support on. Or have some bring over a portable welder to you??