First off my gaming group rotates DMs around and it's getting closer to my turn to run a new campaign. Now previously I've used published materials. (RotRL, Kingmaker, Attack on Myth Drannor and a few other one shots.)
But I uh opened my big mouth about an idea to have a party start at lvl 1 and have the first few levels be about simply surviving AFTER their home city gets flattened by a colossal creature. In fact lvl 1 would be "Hey it's a normal day in your profession...then earthquake...huge roar...and all hell breaks loose."
(Inspired by Attack on Titan, and hordes of kaiju movies, the new Godzilla film, Pacific Rim, Cloverfield, etc.)
What my group liked was the idea of having to collect the parts and spells to build a colossus to protect their country from more kaiju and insanely large monsters.
(most likely going to use the stats for the stone colossus from B4, though it's not going to be just get the spells and gold to do it.)
Where I need the advice is:
1. I have never built a campaign on my own from the ground up. Yes I'm setting this in Golarion but other then that, not much structure, nor experience in how to build a 'by the levels' story campaign.
2. Going by strict CR there will have to be encounters where the party might be level 1-4, but the monster may be waaaaaay higher in CR. The hope being that rather then fighting the city eating monster head on, the challenge is dealing with not getting stepped on, rescuing people, fighting 'smaller' monsters, (like Clovie's parasites for just one example) and dealing with refugees and looters passing in the wake of the monster(s)
Unfortunately I have no idea how to scale such very dangerous but not technically combat challenges to award experience.
So...tldr...How to make/plan out a campaign? Cause I got overly ambitious and stuck my foot in my mouth.