There are a few reasons for wanting rounded cables. The most popular reason is for better air circulation in the case. By reducing the cable size air moves easier past a round cable as opposed to a flat ribbon cable. Other reasons are to stop clutter in a case, and it just looks cool.

After reading other articles about self rounding I found the same thing, use a razor knife to cut between the wires. That method worked fine back when those articles were written, that is when ATA-33 was the standard with only 40 wires in cables. A ATA-100 cable has 80 wires in the same amount of space. If you try using a razor on a ATA-100 cable you better have allot of time and a steady hand or you better have a bunch of extra cables laying around. This is what happens if you don't have a steady hand using a razor. You get exposed bare wires and no one likes bare wires

Instead of using a razor and spending a hour or so on 1 cable, you can finish 2 ATA-100 cables and your floppy cable in under a hour. And most likely not make any mistakes.

First, of course, you're going to need a ATA-100 cable. You'll need a heavy duty needle, I think it's the kind used for yarn. The picture on the left shows the needle size. As you can tell the bottom needle is twice as thick and longer then normal needles I have tried using a smaller needle only to have them snap in half. You will also need a cutting board, tie wraps (about 8-10), and electrical tape. No electrical tape doesn't just come in black, you can buy it in a variety of colors at Home Depot. 

I recommend using a cutting board so you don't cut/scrap your work surface.