HowTo? Fixed / Singlespeed conversion from an old bike

have you read any of this thread?

If you don't use a proper track hub with a lockring, you really should have two hand brakes. If not, a front brake failure followed by a sudden extra effort at "resisting" could break the sprocket loose at the worst possible time, and you'd be toast!

the rotafix method really isn't any better than using a nice quality chainwhip.