What is the best way to educate yourself on how much materials should cost? How do you manage subcontractors without a general contractor and make sure subcontractors are insured and have waivers?

In my experience, unless you are from the construction world trying to GC yourself is a bigger headache than its worth. You can accomplish more by finding a general contractor you can build a trust with, negotiate with them on a high level and let them worry about their own profitability. Create a budget of your full project and work from there. That way, you have a single person to go after if you aren't satisfied with the work or if something goes wrong. Taking on all that liability yourself isn't (in my opinion) worth the money possibly saved. 

I'd like to suggest that saving money on a general contractor can come at great cost.  If you are doing permitted work, there's got to be some GC listed on the permit.  If you get caught going without a permit, it's not usually good for you.  It can be low risk for some things.

The GC also can help with pre-existing relationships with subs, and pre-screening.  Doing permitted work also kinda is a screening device too, because it screens out a bunch of people that aren't licensed or don't do good work.  

My partner is a GC and she's worth her weight in gold just because of the relationships she has with other inspectors in the city.  Food for thought.  To more directly answer your question there's no shortcut.  If you are GC'ing a project and need have GC'ed something before, then yeah it's going to be as difficult as you think.  You will have to develop processing for vetting them, establishing draw schedules, managing change orders.  YOu'll have to learn over time what the market price is for installing what you are requesting.  Getting multiple bids will help with that, but deeper discounts come when you give the same person more work.

I agree with @Bassim Audi and @Peter Halliday . The best way to learn is to hire a GC and work with them.  Whatever you lose in paying the GC is worth the pain and headaches and costs you will have along the way until you learn the ropes.

I think it depends on the scoop of work. Are you remodeling a kitchen or an entire house?

I've taken the hands on approach for most of my properties which forces me to learn the costs. YouTube and Lowes are my best friends. I recently swapped out an AC capacitor and saved at least $150 in labor. Material cost can be calculated but the price contractors pay is usually lower. I agree with the above responses about  permits and why it's important. Managing contractors is not that hard when you find good ones. Be available, answer phone calls, and check as stuff get completed. For some projects hiring a pro is the best course of action.