New to COdeIgniter

For me, a complex project can be split to smaller modules to solve with and you can start by prototyping it.
You need to identify components that will be interwoven in your business domain.

You may find that it is more challenging to cope with business logic changes than writing the code itself but allow
yourself to adapt those changes quickly.

The boss will evaluate on progress and I like this humble advice 'non technical people wants visual'. That means you need to show them visually. A schematic diagram with tables, a mockup app site, roadmap e.t.c then adapt your way of development
I prefer TEST DRIVEN DEVELOPMENT (TDD) and so far it hasnt failed me.

I enjoy building app from ground up and in fact I dont ever think there is a project that is too big to handle!

Regards