I have worked with fresh "Over the border" types, so maybe I can help.

You need to replace the front frame rails, due to damage or rust. You want to know how to do this. Correct?

This is a difficult job. Only a person with COMPETENT welding skills and an adequate working space will be able to do this job right. You will need a perfectly LEVEL floor and the engine, trans and front suspension will have to be removed including the K member. Sound tricky so far?
You will have to remove the seats, the carpet and floor insulation, the seatbelts too. Once you have all these parts spread out all over your neighbors driveway, you can start taking measurements, using a factory service manual as a reference. I think that they used decimal scale, so you will have to convert fractions to decimal "inches" . You will not need to learn French. Once you have all the measurements confirmed and documented in crayon on a napkin from the local taqueria, you can drill out the hundreds of spotwelds that hold the frame rails in place. Installation is the reverse of removal. Okay that was easier than I thought.