My advice to you is to study drawing in this order:
Gesture, Construction, Anatomy, Technique
Gesture is the overall action of the body, you want to capture the energy/essence of the pose. These should be real light and loose
Construction is where you start to lay in the solid forms of the body like the head/torso/hips/arms/legs
Anatomy is when you start defining clearly what's what on the body. This would be where muscles would go.
Technique is all the extra stuff that makes the drawing stand out, shading, texturing, color.
I wouldn't use those "How to draw manga" books as your only reference, but would rather suggest reading books about just drawing in general. Read all kinds of books on drawing in all different styles. It's a lot easier to learn to draw with solid foundation skills and apply it to anime/manga than it is to just draw it manga style only.
You could practice one body part at a time like the previous person said, but the problem is when you want to put it all together...you will struggle unless you're really good with proportions and anatomy.
I was the same way when I started out drawing because all I ever drew was head/shoulder drawings cause I was uncomfortable drawing the whole figure. Well I got really good at drawing heads and shoulders but my full figure drawings were pretty bad.
Above all, the thing I learned most about getting good at drawing is to draw a lot. Practice practice practice. It helps to have a sketchbook, to keep all your drawings together. Don't worry about making every drawing amazing, just get something down on paper. I've learned that sometimes you have to do 10 bad drawings just to get 1 that you're happy with.
Hope some of this helps. I'm studying animation and these are some of the things I've come across/struggled with myself. Good luck!