What is the proper way to vent a dryer through a crawlspace?
The "proper" way to vent the dryer, is to go out the wall.  This could be the wall of the garage, or the wall of the crawlspace. Though if you're venting through the wall of the crawlspace, you'll want to make sure the vent is the proper distance away from any crawlspace vents. 

You're also supposed to use rigid metal duct, not the flexible crap. There are also limits on the vent length, which is reduced by bends in the duct. Check manufacturer installation instructions for vent requirements, and restrictions. 

In the best case, venting into the crawlspace is blowing moisture and lint into the crawlspace. Worst case, you're pumping combustion byproducts into the space as well.