I'm getting ready to build a winter singlespeed with fenders and studded tires--something I can ride on bad frozen roads and hose off and spray with WD40 from time to time without getting obsessive about dirt and salt. Because I have some monster climbs to deal with, I want to use double chainrings and two cogs in back that I can shift manually. Based on parts I've got on hand, I'm thinking of something like a 47 and 36 on the front and a 21 and 32 on back. I know, that makes for a very low bottom gear and a not-very-high high gear. Show some compassion and don't make a big deal about it.
I know I could quite easily do that with a spacers and cogs on a cassette hub--have done it in the past, in fact--but Sheldon talks about doing the same thing with a freewheel:

http://sheldonbrown.org/hercules.html

All of my bikes have freewheels, but I've never had one apart. Generally speaking, how hard would this be to do? I know how to get the cogs off with two chainwhips, but I'm not sure what happens after that. Are the spacers the same thickness as the cogs (I'm talking 5-speed here)? Do you just take off the cogs you don't want, replace them with the same number of spacers, and screw everything back together? Though not entirely clueless in other areas, I am as a babe in the woods on this subject.