I’m assuming this is more geared towards my critter legs than the people legs, since… people legs are people legs. Let me know if I misunderstood!
I’m not sure I have a whole lot to say, but I can make a few notes about the types of things I usually draw the look from, but it’s a fairly simple point A-to-point B I think (though if anyone has specific questions, I’d be happy to answer)!
I use the same “lightning bolt” shape for basically all of my fantasy / anthro legs, because I’ve found this ends up being one of the more believable ways of melding bipedal plantigrade and quadrupedal digitigrade while still looking kind of animalish. This shape can be seen in the hind legs of most unguligrades very easily:
But you can see it in other tetrapods too, as long as you start at the hip and end at the toes:
But you might notice there’s different proportions to those, which have to do with the kind of locomotion the animal is adapted to. For example, the exaggeratedly long distance between ankle and toes in the deer, horse, ostrich are adaptations for a cursorial lifestyle. Playing with these proportions in your design influences how we may view a character. For example of my own, Marco (left) is a much more high-energy character than Alex (right), which I reflect in leg proportions, even with the same basic leg shape:
Those leg proportions also affect what the most natural default stance looks like. As in the above, the left has to have a much more dramatic bend to look natural than the right. A general rule of thumb is the higher the ankle, the more dramatic the resting bend in the knee. This isn’t a hard rule by any means, but it tends to help me make things look like they’re neither squatting nor about to fall over.
There’s a lot you can play with and I always like reading into things, so it may help to look up which muscles do what and what shapes come out of those. Look up plenty of critters and look at how their legs are set up, there’s a lot more base shapes to play with than my “lightning bolt”!