When the flip-flop hub and fixed gear cog arrived in the mail, there was one more thing to do. I took the spokes and old hub off the the rim that I planned to use, and reset the new hub in the wheel, relacing the spokes so that the new hub was now mounted inside the old rim.
Potentially a person who converts a regular bike into a fixie will also have to have the rear wheel "redished" which means adding spacers to make the fixed gear cog line up with the crank so that you have a good chain line. Because if you think about it, the fixed gear cog screws directly onto your hub, much closer to the inside center of your rear wheel than the gears were before.
Normally, your rear wheel is just a little off-set to the left, to account for all of the sprockets on your rear cassette. When that cassette is gone, all the space that it previously took up often needs to be filled in, otherwise your chain will run in a crazy crooked line from your front chainring to the rear cog. When you look at the chain line from the rear, you want the rear cog to be directly in line with the front chain ring. If it's off, you'll have excessive wear on the chain, chain ring and cog, not to mention lessened pedal effeciency. Potentially you could have problems throwing the chain too. Mine was good as it was on the Raleigh, so fortunately I didn't need to redish the wheel. But that's not a big deal; something your bike shop can easily do.