Step 4: Null modem interface
You can get null modem adaptors readily made so you do not have to make one. I have left the specifications in case you want to make your own or you do have access to the parts, but not the ready made adaptors. A null modem is a way to interface two serial ports so that they can talk to each other. This sometimes known as a hard wired connection, See also http://www.lammertbies.nl/comm/info/RS-232_null_modem.html
Note: if you are unsure about the connections, then get a professional to help. I will not be responsible for any issues. Also some devices use non-standard rs232 pin outs. you have to check the documentation for each device to make sure.