i just need to know how to build this thing!
You just did, didn't you? Be a Broodmaster, and give the Eidolons the Craft (Siege Engines) skill. I'm not sure the rules will allow this to be as effective as it might have been though.
- First of all there's no real reason to craft your own Siege Engines - just buy them from an NPC.
- Siege Engines, by their nature, are big and heavy. Making one is slow and laborious already. But moving them over long distances? That's a logistical tour de force.
- Most Pathfinder games are, in military terms, skirmishes. Siege Engines are specialised in, you guessed it: sieges. They'll simply not be very useful, even the ones that can be used as anti-personnel artillery.
Broodmasters are effective already, in their speciality: Action Economy. You're putting a lot of miniatures on the play mat, so the best way to be efficient is to look at squad-based tactics. Most parties will appreciate a good "shield wall" in front of them. Or "volley fire" support. Or give everyone in the party their own bodyguard/flanking buddy. Those are tactics that might not give quite the same spectacular mental image as a fully crewed Siege Engine at work, but they're tried and true.