I've never done this conversion but I always assumed you just take off one shifter, the front derailer and the chainrings, then you put on a single chainring of your choice on the inside of the spider? If you don't have a chain guide before the conversion, why would you need one after? Or are you going to use the bike for downhill afterwards?

I actually did this conversion on my road bike that I made fixed gear, come to think of it.