I picture that after Cthulhu has risen. And assuming his enemies are busy else where in the universe. That he would probably just chill on his island city for awhile. Us cultist who were not present in the pacific ocean, upon his awakening, will probably be indoors, using magic & weaponry to hold off the masses out on the streets.(unless at this time the COC has temples for us to hold up in). But this may not be needed. For I don't think that Cthulhu causes the madness we'll all experience. I think he merely unlocks our truest selves.
So for cultist who've already crossed that boundary, I would think it would open us up to greater power. At least it should. So with that in mind, we might be protected by our own power. Or just a straight up 'marking' from Cthulhu that makes us seem terrifying to others(Like in Batman begins). All the while, Cthulhu just sitting back, absorbing all the crazed energy the masses give off.
When our high priest returns from cthulhu city(either by the same boat he went out on, or flying through the air!!) Id think his job would be to contact all his followers around the globe. This would most likely be done through telepathy or teleportation. The highest ranking cultist in each country(or state) would be given a set of instructions.
#1 Unite all members.(using telepathy) & have them meet at the capital or greatest city.(mine would be L.A.) Doing this on foot would be dangerous, but perhaps our magic would protect us(make us invisible to the masses). This faze could take a few weeks.
#2 Establish a temple. This would either be the capital building or a stadium. Preferably the biggest building you could find. This faze would take only a few days.
#3 Cull the masses. Used for sacrifice & the building of the temple. Winged cthulhu creatures could help with intimidation. Since there's no food, the masses could eat each other. We cultists could eat the pick of the litter, or just have the slaves find us food. This faze could take months if not a year or more.
#4 Raise the Ancients Ones. Continuing our sacrifices since faze #3, more of Cthulhu's offspring will be awoke & roaming about. This faze takes years(three or more). By this time Cthulhu is up & about.
#5 Total conquest. By now the COC is the only operating government on the planet. Those slaves that were sane enough to be of use, might now be allowed to join. For food i imagine there is a slime we might eat from our temples that give's us sustenance. Perhaps thats how we keep the masses in line.
In this faze, we(with the help of his offspring) will construct monoliths around the world that Cthulhu will use for his own cosmic desires. Finishing the job he came here to do in the first place(I.E. convert the milky way into a massive black hole, fire off a super nova chain reaction that pulls the milky way into another dimension(his home realm? etc.etc.)
#6 Ascension. The COC schools all humanity in the ways of darkness. Which at this point might appear like the holiest thing ever. I see a ceremony where (like in Logans Run) we all gather to watch those who are able to ascend. We chant & cheer them as one after another their bodies are picked apart by tentacles & slime. If they were ready, they either burst into a cthulhu type creature of their own or we watch there souls gloriously ascend & merge with with the green bale fire light coming from the floor (or ceiling) of the temple. Life goes on for hopefully a long time like this. Finally as a race we are unobstructed from the bullshit that's been in our way since time began. Now only ascension matters.
#7 Cthulhu splits. I see him more interested in his original mission,than in us. We just gave him some good entertainment while he was here. But his goals are cosmic. This planets position in the galaxy is what he's really interested in. I wouldn't be surprised to find out that the other planets hes been to are apart of his galactic plan(what ever that may be) & that its to achieve something on a massive scale. And I mean galacticaly massive(if thats even a word). Anyway, tell me what you all think.
Ia Ia Cthulhu Fhtagn!