Your proportions are going to be all over the place at first, that's okay. Don't get yourself caught up in trying to get it 'perfect'. You're not there yet. What you want to do is capture the 'gesture' of the image. You're training your eye to feel the subject and to put it down as quickly as possible using sweeping gestures.
Here's an exercise for a total beginner: go to a park or a cafe with a ream of paper and a big fat chunk of charcoal. Sit and start people watching. Each person that catches your eye, quickly put down a a squiggle, yes filling up a 3rd to a full page each, that captures the 'feel' of the shape their body makes. Maybe just a head and a curving spine here, a bent blob there. Are their shoulders hunched over, looking at their phone? Are they fat, tall, skinny? Old? Youthful? What's their energy like? Are they trying to hide from the rain? Are they skipping? Holding hands? Striding triumphantly? Depressed? You want to get the idea of this on the paper as fast as possible without lifting your drawing utensil. You're training your eye to see the whole person, rather than the parts.
Do 20-40 of these as a warm up. Might take ya say 15 minutes or so. Maybe longer at first. I like to bring my sketchbook with me everywhere and do it when I'm sitting down. You could trying going to the park on the weekends and switch to the cafe with a good view of the sidewalk if the the weather is incliment, or go to different places so you can capture different types of people doing different things. Probably you're gonna see a lot of people walking, not doing anything interesting. In time you'll begin to see the subtle nuances and capture it more vividly with just scribbles.
Here's some examples I threw together.
Remember, don't life your pencil! You're trying to capture the whole imagine in a scribble! You want to train your eye to see the whole thing all at once, because that's what give you the sense of perspective. If you're looking too hard at one part, drawing it seperately, it's gonna be out of balance from the rest of the image.
Another exercise is drawing stick figures. An oval for the head, two smaller ovals for the shoulders, a wide oval for the waist, then trace a line that follows of the curve of the backbone. ovals for the elbow, wrist, knee, and ankle joints, each connected with a curving line for the limbs. Set the figure study tool to 30 sec intervals and do it for thirty minutes. Just bust 'em out one after the other.
Remember: don't get attached to any particular subject. Focus on getting the whole thing down as quickly as possible. If you don't finish just move on. In time your eye will start to fill in the blanks. The more your draw the better you get. It's better if your images are larger--it's easier to scale down than scale up.