How do you prune/ cut back a severely overgrown crepe myrtle?

http://homeguides.sfgate.com/controlling-crepe-myrtle-height-pruning-37014.html

http://www.southernliving.com/garden/grumpy-gardener/what-concerns-p

@Roxaneg same article I was gonna to reply with. For an amusing follow-up:

http://www.southernliving.com/home-garden/gardens/crepe-murder

Here in Georgia we see the worst forms of crepe murder!

Not sure what you mean by 'overgrown'. Crape Myrtles are like any other tree; they will grow to their mature height. It's like asking how to prune a 'severely overgrown' oak tree! Here in Tallahassee we call it CREPE MURDER. People whack off the limbs of the tree down to stubs every year. Would you do this to an oak, elm, magnolia, etc.? NO, you wouldn't. They do this in the misguided belief in the myth that it MUST be done to get more blooms. Nothing could be farther from the truth. Crepe Murder weakens the tree, makes it look freakishly ugly, and makes the owner look foolish.

I think what your asking is about the suckers that spring up around the base and on the main trunks of your plants. Crepe Myrtles are more a bush than a tree so if you want cleaner more defined plant you take the suckers off.

Suckers you ask? The little sprouts that come up around and on the plant that aren't where you want them. No, you shouldn't cut the plant back to the ground or even take all the branches off the trunks. If your in the South they will grow back but look awful until they do and they don't help the plant. I was always taught that you trim back about a third of the plant taking out cross branches or damaged wood and those pesky suckers! I am training my Crepe Myrtles to be trees in this fashion and it is slow because they are only two years old but it is working well.

You can remove the small trunks (suckers that were left to grow). Or you can remove all but a couple of main trunks and go from there. 15 feet is likely the mature height of the variety you have. Pruning a crape myrtle is the same as any other tree. Prune to remove old, diseased wood or branches that are crossing or rubbing against each other. Also, remove 'suckers'.

Hope that helps!