Where to stand - Adds will be entering from the left and right sides of the room, so setting up shop in the middle of the room is a good idea.
What to watch for -
1. Death and Decay! - Lady Deathwhisper will toss this AoE spell at the feet of a random toon throughout the fight. This green rune is a little bit smaller than the circles of death you find in Ulduar: Iron Council, but they're still fairly large and not something you want to take your time moving out of. Death and Decay ticks for roughly 4500 dmg per second. The usual drill applies here... move first, cast second.
2. Adds! - Six adds of two different types will spawn at regular intervals throughout phase 1. One type (cult adherents) will be attacked by melee, the other type (cult fanatics) will be attack by ranged. Shortly after death, both types will reanimate and will have to be killed again. Luckily, picking up, separating, switching to and from these adds is none of the healers' concern. All you have to worry about is not getting too close to them and not gaining aggro when they first spawn or reanimate.
Note: If you'd like to see a list of all their abilities check out mmo-champion.com.
3. Shadow Bolt! - This nasty little spell can hit for over 10k dmg, so be ready to heal some hefty damage spikes.
4. Dominate Mind! (25 man only) - Lady Death will mind control one player, increasing their damage by 200% and healing by 500%. Feel free to CC one of your buddies if you see that their mind has been taken over.
5. Curse of Torpor. - This curse will add 15 seconds to the cooldowns used by anyone afflicted with it. In the grand scheme of things this curse isn't much to worry about, but still, it can (and should) be removed with your remove curse spell whenever possible.
Where to stand - The middle of the room is still a good place to be as long as there's no Death and Decay under your feet.
What to watch for -
1. Shadow Bolt! - This spell is a whole lot worse in phase 2 than it was in phase 1. Reportedly it can hit for upwards of 40k, so watch out! The good news is that it's a two second cast that can be kicked. As long as your tanks and melee are on the ball, you should never see one of these cast in phase 2.
2. Death and Decay! - Little Miss Death will still cast her DnD during phase 2, so stay on your toes. It's the exact same spell as in phase 1.
3. Dominate Mind! (25 man only) - In phase 2, Deathwhisper can mind control up to three players. The damage and healing bonuses are the same as in phase 1, and CC will be even more important during phase 2.
4. Vengeful Shades! - The boss will run around the room pulling Vengeful shades from the bodies of fallen cultists. These shades will explode if they catch up to their target, so feel free to take a page from Monty Python and "Run away! Run away!"
This fight is quite a bit easier than Lord Marrowgar, and not very healer intensive. Stay out of DnD and far away from adds, and you should have no trouble at all healing through this.