The described project is for one-way or two-way communication using (i. e. transciever). If you know you will only send commands "blindly" you can attach a transmitter to the PC end and a receiver to the Arduino. Those modules are really cheap.
Step 1: What you need
- USB serial adapter
- Two or more Wireless RS232 modules (e. g. EWRF 3022)
- Arduino. It's a Nano in the picture but any model should due.
- A gadget box
- Soldering equipment
- Basic Arduino skills