That is close but I am not sure it is correct. I believe the rangers have a factory amp so it is not quite as easy as that.
Go to each speaker and cut the wire and attach new then route those to the amp. If you want to use your stock radio you will need a couple of "Line Output Converters" and those should be attached to the speaker wires directly after the head unit, then run the RCA cables from there to the amp. You will also need to tap into your hot when cranking power line (yellow/black I believe for your truck) and then run this to the remote connection on the amp so it turns on when the truck does, the same time as the head unit.
also remember that your stock speakers are not made to handle much more than the stock system, plus even if they do handle it they will not sound much better as they are just cheap speakers with cardboard cones. You could go online and ored 2 pairs of 6x8 or 5x7 speakers for under $100 and they would sound way better than the stock speakers, even if they were not amped. If I were you and I planned on spendng any kind of money on an amp, I would spend the money to get some speakers that could handle it and sound nice first.
4 of these wouldnt sound to bad, my friend installed them to the factory amp and they sound way better than stock. http://www.ebay.com/itm/Kenwood-KFC-...ht_2252wt_1139
just my 2 cents but there you go.