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Once you have done all of that, simply hit the Check In button (not in this screen; was too slow to screenshot it) but you will see that all of the people tagged, including yourself, have been checked into the location that you spoofed. Ta da!
Please read the disclaimer! You can find this and more, including the explanation - as well as the pro's and con's of using spoofed geolocation on the ZDNet iGeneration blog.