I'm converting my old favorit bicycle to a fixie, but the hub is obviously not made for a fixed gear cog. I've removed the freewheel (not sure if it's called that, the thing that spins backwards on the hub..) but I'm left with a "screw on hub" (the cassette with the freewheel was screwed on to the hub). How can I install a fixed gear cog on that? Is there even a way? please help..
I've not done this, but I beleive the threads are compatible with a standard fixed gear cog. If so, just buy a cog and install it. Then put a lockring from an adjustable bottom bracket on as a lockring (it has the same threads as the hub).
Note that this will not works as well as a proper fixie hub, most obviously because it's missing the left hand threaded section that normally holds the fixie lockring in place. I strongly suggest using locktite, or possibly even superglue, to hold the cog in place. Otherwise when you back pedal to brake there's a real danger that the cog will undo and you'll lose drive.
There's a detailed explanation here that covers what you've done plus the next steps.