Hi
First I would say the ref photo did not help you because the black tight (any black clothing) flatens the form, i.e. You can no longer see where there are muscles undulating under the skin. Compare her bare torso with her legs and you'll see what I mean. Then this model is too skinny for someone who is beginning, try to find a more fleshed out model for the time being, you'll find it easier. Not that very skinny models are no good, they are just more challenging. Make it easier on yourself for now.
The other problem is artistic anatomy. You need to know the points of insertion of muscles and draw the slight bulge of each muscle until it connects to the next muscle or bone by a tendon. For instance from the shoulder you'll draw the deltoid until it meets with the biceps in the front or the triceps in the back, and so on along the body, same thing for the torso and the legs.
Third point: the bulges formed by muscles on either sides of the arms and legs are never opposite each other but in diagonal to each other. They also never go in the same direction. If you look at a master drawing you'll see that clearly. When the muscle shapes are properly placed you can a draw a zigzag line inside an arm tracing from bulge to bulge.
Sorry if this is hard to understand, but it's the best I can do without drawing it for you.
Congratulations on your attemps, it's hard at the beginning, dont give up if you find it difficult, you'll eventually get the hang of it. Practice with books or some youtube videos.
Best to you
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