The MAX232 is just buffering signals, no code changes needed.
The chip changes the uC's Tx 0/5V output to ~+10v/-10V to send down the wire, on the receive side the chip changes the +10V/-10V back to 0/5V to drive the uC's Rx pin.
So you need a MAX232 on each end, and five 0.1uF caps, or 1uF caps, depending on which driver you get, for each chip to make the +/-10V for the driver.
http://datasheets.maxim-ic.com/en/ds/MAX220-MAX249.pdf