frame rail replacement is not really for a novice
but it is not impossible for a novice to learn how.
you are getting in as deep as you can go..
do you have any welding experiance?
own a welder?
if not,get yourself a decient entry level machine
and start practicing before you go near a car.
your best fitting replacement metal will come from other donar cars
do you have the parts you need to replace?
if not, start hunting...
when you have a large area to replace like 2 rear rails
your best way to attack it is to replace what is bad
by what is called a clip.
you would keep the donar rails together as a unit and replace both rails at the same time.
the more parts you keep together from the factory
the more likely everything will line up when you put it back together.
also you will need to learn how to extract parts without
hacking them up.
you will also need a good spotweld drillbit
for that task.
good luck,you have a very large hill to climb!
keep asking questions on here will be your best help..