Just made a DK as an alt and perhaps a future main if I prefer it to my Mage. Right now sitting at level 60 and I've done 2 dungeons, but I am not too sure of what my rotations are. I have had tanking experience with a Prot Warrior to level 69.
Here's my current order when tanking:
1. Death and Decay on area as I pull mobs.
2. Put up diseases and spread to all targets with Pestilence.
3. Use Blood boil or Heart Strike depending on how many mobs there are.
4. Death Coil to dump Runic Power.
Is what I'm doing right? I find that while I'm getting the diseases up on 1 target and about to spread to the rest, some mobs go on to attack the DPS or Healer, so I'm unsure whether I should use Blood Boil first to get enough threat.
Do you have Rune Strike? Use that over Death Coil. I don't think that what you're doing is bad otherwise, but how you tank will change over the next 25 levels. Make sure you're using Death Strike with healthy frequency and yes, try and grab the adds attention with something before you get healing aggro or dps going after things that are not marked (did I mention you should mark up a kill order to get into that habit?). It can be Blood Boil or DnD but be sure to follow up with your other abilities.
What you are doing is correct, except the death coil dump, it should be rune strike. Later on you'll get more abilities that will change tanking completely as it is right now for you. dnd should take care of threat on mobs that aren't the primary target but if you see you are losing a particular mob tab target it and throw in a rune strike at it.
Look out for dnd and mob pathing so u make sure they walk into it. Also I may suggest an addon called tidyplates that pretty much changes how nameplates are and has options to include color changing depending on wheter you have aggro or not and you can also add a threat bar, so you can easily spot what mobs you have aggro on and which are loose
DK tanking is awesome. You really need to get the threat generation from bb and or dnd before spreading diseases. Frost fever from icy touch is your best opening disease as the "slowing" ability then "blood/scarlet fever for the damage reduction and dps increase, the increase will help your threat and the groups overall dps. There really isn't a perfect or set rotation, I have learned from this site, other forums and trial and error, its about priorities. Your #1 priority as a dk tank is your threat. Hope this helps. PM me if you have any questions I may be able to help you with. I have bookmarks for posts on here as well as elsewhere.