How to go about organizing an extremely large project?

Have you looked at Project Management software? That allows you to chart a critical path, and to hang as much stuff of of it as you need to.

Gantt Charts help organize projects, set timelines and, as @marinelife suggested, develop critical paths. There are quite a few programs out there to work with. We used to do it by hand, still do for smaller projects but software helps.

Break your project into functional parts and then examine each of those for individual tasks needed to make it happen. Have a separate list of questions that need to be answered for each functional part. Have the questions separated helps because you can work out those answers when you get there.

If we take your global phone book example, you might break it down into functional parts as

> data gathering
> storage
> using the information

And you’d break those down into tasks, if we looked at data gathering an incomplete list of tasks might be

> find existing providers of phone books
> figure costs of acquiring their data
> figure costs of gathering data on our own
> contrast both

And some example questions for data gathering might be

> what specific data fields do we need? First name, last name, phone number, address, etc.
> Are these going to be used universally? Regional differences between phone number and address formats will need to be addressed
> How will we handle people with more than one phone, is the goal to get one number for each person or to catalog every phone number?

By breaking it down eventually you end up with actionable tasks that you need to get done. Those tasks are usually easier to track and complete than just standing back and looking at the whole picture, which can seem insurmountable.

http://www.edrawsoft.com/PERT-Chart.php

Make the best outline you can of the broad strokes that need to be accomplished. For now try to avoid becoming bogged down in detail. If you’re planning a million-volume library then the last things on your mind are architectural details, moldings, carpet colors, etc. for the building. But it might be germane to consider how many employees and visitors you’ll have so that you can plan sufficient rest rooms, potable water, HVAC, etc.

For example, if it were a building project such as you suggested, you’d need to acquire land, arrange financing, provide transportation for materials and workers (if the transportation doesn’t already exist – we once had to make a construction estimate for a power plant on an island in the Indonesian archipelago in which the specification stated that “access to the site is currently by canoe only; a road is planned… {sometime}”) and “create a specification for the building” as an activity.

Part of that specification will involve engineering and scaling the project.

As you start to organize the steps, then you’ll start putting them in sequence and (eventually) determining the sub-steps to accomplish the major ones. For example, in order to write a coherent building specification, you’ll need to hire or enlist some civil engineers to work with you to determine the suitability and carrying capacity for the site you’ve selected, or even help you with site selection.

As @marinelife suggested, Project Management software is all about this. As you determine the steps and sequence of activities, then you start to estimate time to accomplish each step, start logic (which tasks have to be complete to start succeeding tasks, and which ones can be started in parallel, and so forth) and what resources will be needed (and costs incurred) to complete each one.

First you need to arrange the financing, usually. You need a backer, investor, or a lot of your own money “at risk” to start the planning, scheduling, and specifying (engineering) process.

Talk to a consultant who has already led a project of this size.