If you're willing to just wrap the chain around one gear and leave the others, you could probably get away with just shortening the chain and removing the derailleurs. If you don't pick the gear that lines up with your chainring, the chain will come off more than you'd like. In general, you probably won't be happy with the results.
To see what's involved in a more thorough conversion, look here:
http://sheldonbrown.com/deakins/how-...onversion.html
I know it says "fixed gear" and you just want singlespeed, but most of the concepts are the same.
For more detail, go here:
http://sheldonbrown.com/fixed-conversion.html