1) put tanking gear on.
2) stay in defensive stance, spam revenge/sunder/shield bash (yes, shield bash. it has 300+ bonus threat.), spam tclap if multimob tanking, keep demo shout, tclap and shield block up.
you will be fine tanking regular instances as fury as long as you have the gear for it. i offtank trash in kara, gruul, mag, SSC and TK while dps specced (fury or arms). my only problem is that my mob can't be the first to die, as my threat generation is kak compared to a prot-specced warrior.