I've got this REALLY AWESOME Iron Kingdoms 5e setting book from Privateer Press. I'm trying to data-entry custom races into D&D Beyond's homebrew system for use in my upcoming campaign, but I'm staggered by its complexity.

Is there a manual that I can review?

I don't know about an official one for races.  The customer service section doesn't have a guide for homebrewing races.  One of the Mods, Davyd has written some pretty good tutorials for magic items, and a few other aspects of homebrew, don't know if he's done races.  I would encourage you to check out the homebrew subforum of this board and start a thread on the first race you want to do and you might get some folks willing to tutor, and return to it as need as you build out the rest of the book. 

We have some tutorial articles that deep dive making various topics:

• Monsters
• Magic Items
• Updating Homebrew

I know Cameron and myself had a great time writing them, so I would expect more in the future. The dream is to cover each element of homebrew in depth.

I found the best way to learn was to find something on here that did something similar to what you want and make a copy. This will let you see how it was done.

Making a proper guide is a challenge because the homebrew system has a lot of options and modifiers. It would be insanely long to cover in depth. The HB system isn't the most user-friendly and there some quirks that are illogically made. It's a clunky system. 

Now, it's not so bad at all when you get used to it and you've had a bit of a play around with it. But even so, to try and explain things in any concise way is.. challenging. Even the guides Davyd posted are relatively basic compared to what those makers can actually do.

I know Stormknight was going to try and make guides but those plans fell through -- too much to cover, too little time.

So, yeah. Best to just make copies and play with it a bit.