For this step you will need a ps/2 mini din, full size (xt) din, or usb jack (only some usb keyboards are backwards compatible). Connect it to an Arduino using 4 wires, for Interrupt (Clk - D3 on the arduino or pin 5 on the atmel328P-PU), Data (D4 or pin 6) , +5v, and Gnd.
Download the keyboard library
and upload the TX sketch to the Arduino. Check to see that you are reading the keystrokes in the serial monitor.
The schematic shows D5 as the TX port to the host Arduino, but we will be using D10 (Atmel pin 16) eventually. Just make sure you edit the code for whatever digital pin you choose to use.