I had a M-Audio USB uno, which worked great on my ghetto keyboard, but on my newer setup, it would constantly dump garbage notes. Rather than go spend yet another wad of hard earned money, I realized that I probably had most of the parts at my house already.
Step 1: Do your homework
My local Fry's Electronics didn't carry a 6N138, but they had the NTE equivalent which I found was a NTE3093. I had resistors at home, but I found I was out of 220 ohm, so I went with 270 ohm, which appear to work just as well.
I didn't care about sending MIDI signals out of my PC, so I left that part out of my design. I wanted all the parts to fit within a DB15 casing.
Step 2: Make it REALLY small
To make this go a little easier, I broke off the 3 extra pins from the IC.