People who draw manga, have usually learned to draw the "traditional way" first. The ability to draw people from many perspectives and directions in black and white in 2D; you have to first understand, how things look from all directions and how you get the shapes look real (soft, round, convex) in 2D. How the shadows fall on surfaces, how items go behind other items.
I took art school for 12 years weekly as a kid, and probably half the time we just had charcoal pencils and we drew everyday items and things and people over and over again. Model sitting on table, model standing, flowers, coffee mugs, sceneries from the window.... I swear I have drawn one particular coffee pot at least 500 times in my life.
That's how the coordination with hand, eyes, and brains will develop. There is a saying that you can master something after practicing it 10,000 hours. Here is a good practice, that you can do on your own:
- pick an object, or a friend who's co-operative, or watch out the window and pick maybe a car or so, that's staying in place. Or build a small item scenery on your table: a clock, book, flower, toy.
- draw the object(s) with soft pencil on cheap paper in say, 10 minutes
- next paper, draw the same in 2 minutes.
- next paper, draw the same in one minute.
- next paper, draw in 10 seconds.
- next paper, draw in 5 seconds.
- next paper, draw in 2 seconds.
- destroy the papers, re-arrange items, and start again.
The purpose of this is, you start to understand how everything forms and how forms are connected to each other. Once you have only few seconds to draw, you have to understand the shape of things, to be able to draw it with few lines. So repeat this as often as you just can

and the papers that you draw here, don't get attached to them, think those same way as a piano player has to practice every day, but they do not record every practise hour they play. They serve a purpose just at the time you draw them.
Eventually this practice will start to kick backwards when your hand and brain learns to co-operate, and you can draw lines on 2D manga characters that look moving or fluffy or weak or strong. The lineweight makes the 2D-drawings alive, and only way you can learn it is by drawing a LOT. And when you want to draw manga, buy a big roll of skiss-sketch-paper -it's very thin and cheap architecture drawing kinda paper, sold on paperstores, slightly see-trough like oven paper, so you can sketch manga and draw them again and again untill you're happy with them. First try doesn't have to be the final drawing, draw things as many times as you get them right. Use the roll without thinking how long it will last. The faster it goes out, the faster you're learning

Oh, and once you master this, you will also win everyone in Pictionary and other draw-and-guess-games