With positioning dealt with and a clear strategy for the spores established, fighting Festergut is very simple. One of the tanks should pull Festergut and position him in the center of the room. Anyone who isn't one of the lucky eight that get to stand at range (the non-active tank, melee, healers, any extra ranged) stand in a group behind the boss.
A few seconds after the initial pull, orange-brown gas will flood the room. The entire raid will begin to take significant damage from it and heavy raid healing will be necessary. Each time Festergut inhales some of the gas (up to three times, after which he exhales it back out as the Pungeant Blight) the raid takes less damage and the active tank takes significantly more. So there should be a natural flow of healing with progressively more being focused on the tank until the third inhale, when all healing is on the tank except for the very minor raid healing necessary for those hit by Vile Gas or the small dot from a spore popping.
Festergut also puts a buff/debuff on the tank called Gastric Bloat. Each time its cast, the tank will take a spike of damage. Gastric Bloat also makes the tank do 10% more damage with each stack. After the ninth stack is cast the tanks have to swap; a tenth stack of Gastric Bloat instantly kills the tank and does a significant AoE. By the time the second tank is up to nine stacks, the first tank's stacks will have fallen off and he or she will be able to taunt back.
Note: The tank with nine stacks of Gastric Bloat will have to be very careful about threat since their damage output has almost doubled. Paladin tanks, for example, should turn off Righteous Fury and apply Hand of Salvation to themselves.
Three times before each exhale, Festergut will cast three spores on random members of the raid. The strategy for spores has already been covered; follow it, make certain that all raid members get their inoculation buff, and everyone should survive.
As each exhale is coming, healers should anticipate an immediate spike of raid-wide damage and heal accordingly.
There are only ever two exhales in the fight; Festergut enrages before he could possibly do a third. However, after the second exhale, spores are still produced. At that point, they can be safely ignored since the buff they apply is useless. The only consideration is that if there are two spores in melee, one should move out temporarily to prevent the dot from being applied twice. After the second exhale, ranged does not have to move again.
Bloodlust/Heroism can really be used at any point in the fight, at the discretion of the raid leader. The most efficient time to use it is after the second exhale, since the spores can then be ignored and the ranged group will be able to benefit from it more at that point.