DK tanking is relatively easy, since their basic rotation is the same as DPS, and you have a instant ability to gain a lot of threat quickly with Rune strike, but you get it at lv67, so Threat management in BC dungeon 58-66 will be moderately difficult compared to the other tanking classes.

Other then that, its simple, just put your Diseases on your target with Icy Touch and Plague Strike (use outbreak after lv81), hit pestilence to spread them around on multi-target pulls, use blood boil on multi-target pulls, use Death and Decay on Multi-target pulls, use rune-strike when you dodge or parry (keep this key-bound to a button you can hit easily on reflex, I use the ~ tilde key so i can tap it with my pinky), make sure you have scarlet fever spec'd, as it will reduce incoming physical damage from all targets afflicted with blood plague, and when you're not all that, just follow this rotation: Icy Touch, plague strike, heart strike heart strike, death strike (keep Rune tap macro'd so you can get easy death runes to use death strike in panic situations).