There are at least two types of dragon legs. One, the reptile-type legs,
with lots of thick, folded skin showing little muscle. Two, the animal-type
legs, with tight skin showing lots of muscle. Just like the wings, you
can always go for your own fantasy-type or a hybrid between reptile and
animal look. Experiment!
If you want the reptile look, look at some pictures of reptiles to see
where the skin folds and how it covers the leg.
Modeling the animal look is basically the same as modeling human legs,
with two major differences. One, the part between the heel and the toes (hind)
or the wrist and fingers (front) is elongated. Two, exaggerate the muscles.
On the picture on the right is the hind leg of the dragon I'm working on.
I've chosen the muscle look, but I might add scales later on to make it
more reptile-like. Below are some sketches to show you the muscles (yes, I have to create better ones, the one on the right is terrible).