Line

For instance if it rusted through in one spot, all of the line is likely suspect for the same thing (VERY COMMON) In that case you would be better off replacing the entire brake line from front to rear, the rear rubber line and the rear brake lines to both sides. Then bleeding the entire brake system.

You would need the proper sized line, the proper end fittings and a good double flaring tool. Add in the flare wrenches and brake tools as well if you don't already have them.
As for the steps, bend the line to fit properly, cut the line square and deburr it very well, then add the proper fitting and double flare the line. Install the line and repeat the process on the other end. If your replacing only a small section you will be doing this process 4 times. (Twice for each end)