Cut the old one off, weld the new one on.
. Seriously that is like asking how to do a nice paint job in one paragraph. I suggest you lurk here for a while and read every thing you can about body work. Also you might purchase some good body shop books I'm sure will be suggested further down in this thread. There are some very good resources in this forum that can offer some valuable tips and photos. The work involved can run the gamut from fairly sophisticated to a real bugger.
Vince