It is very cheap to buy that cable but if you already have the scrap parts needed..
Step 1: Parts & Tools Needed...
1 x Female Din9 serial connector (DB9)
1 x Audio stereo jack (I take out one of my old pc front panel..)
1 x Audio stereo cable Male to Male
Hot Glue Gun (optional. I use instant Glue)
Step 2: Prepare Components
Identify the pins of the jack with a multimeter..
Step 3: Time to Solder Them Toggether & Measure It..
1 photo 1.000.000 words..
Use Continue Tester to check for bad soldering or short out..
Remember: Shaft = Orange(Rxd), Ring = Red(Txd), Tip = Brown(GND)