nuts and bolts

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun plural The basic working components or practical aspects.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic The basic, inner workings of something; the fundamentals or basics; that which makes something operate, on a basic level.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun detailed practical information about how something works or how something can be accomplished