How do you put 'hair' on your crocheted dog? It's a lot like hooking a rug. You can even use a latchhook to do it with. Or, if you prefer, you can put the 'hair' on with a crochet hook just like you would put fringe on an afghan. The only thing different would be you use one strand of yarn.
Cut the yarn into different lengths for a shaggy look. For a more uniform appearance you would cut them all the same length. You can just add a 'hair' wherever you want it until you feel the area is as 'hairy' as you want it to be.