Squares. Well, scribbled squares. Also scribbled rectangles.
If you scribble a square, odds are it’s not at all square. Fortunately you don’t care because your palm isn’t a square with dead on 90 degree angles. But in a lot of poses a scribbled square will get across the action the palm is doing so a line or three can get across some of the finger and thumb gesture.
Feet, same deal. Your foot definitely isn’t a rectangle. But a scribbled rectangle can get some of the idea across. And the toes or heel will take a line or two.
For both scribbles, the goal isn’t an accurate square or rectangle, it’s to scribble in something that conveys the gesture and allows you to add other gestures to make the idea clearer.
I actually find it’s easier to work on in full length figures classes. Just making it a goal to include the hands and feet in a full length figure helps a lot with making classes focused on hands and feet feel easy.
The hand and feet, is not to hard if you know the structure and anatomy. My tip is for you study the structure and anatomy, if you know that, you can represent as you want! It's very important to know the bones, muscles and action of them.
I'm not taking into consideration the fundamentals of drawing, this is like an alphabet when you started writing your life.
Perspective 1, 2, 3 point.
Shape: (cube, sphere, cone, cilynders etc.)
Plans: (square, ellipses, triangle etc ...). And proportion of head and body, simple muscle groups, light and shadow as well.
I recommend you searching for Drawabox on youtube. ^^