How to keep facial proportions?

I understand facial proportions in the sense that I know where to place things on a face but I don't understand how to keep their size in relation to one another? Any advice on how to learn this? Or maybe where I can learn this? Or perhaps you can just tell me how to do it yourself lol. Just help please.

You could measure with a stick or your pencil by holding it at arms length in front of the face you are drawing. Squinting through one eye- measure what you want to draw by placing the end of the stick at the edge of the feature and placing your thumb on the stick where that feature ends. For example an eye ,the outside corners of the mouth, or the length of a nose. Then keeping your thumb on the measurement move the stick to see how the size of the eye relates to the size of the mouth or the spaces between features. For example you can count how many eye lengths are across the bridge of the nose( usually one) or across the temple to the hair line. Or turning your stick horizontally (still keeping thumb in spot of eye length measurement) see where the length measurement of the eye relates to the width of the eye. By keeping one true measurement and comparing that size to other features they should come out proportionately.
Hope this wasn't too confusing.

I'd recommend a couple of books. "Drawing the Human Head", by Burne Hogarth, and "Drawing the Head and Hands", by Andrew Loomis. I also found this on the internet. It does a pretty good job of showing basic head proportions.