Use an x-acto knife to carefully cut a small hole at one end of the ribbon. Then slowly peel the wire away from the rest of them making sure you don't rip the insulation. I remember someone saying once that you should cut ATA66/100 wires in pairs, but I don't remember why; maybe someone else does. Anyway, when you're done then start twisting the cut wires until they form a nice tight bundle. Then use cable ties or those wire bundling wrapper things (sorry, forgot the techinically correct name) to hold everything together. Don't forget when you twist the cable that it will shorten it slightly, so get ribbons that are a little longer than you need. Personally, I just fold all of my cables and run them along the sides of my case. In fact you can't really even see them when you open my case since they're kind of hidden.
Good luck,
Shooters