Hey Avşin, This is a good exercise to practice to give cloth form. Your drawing is reading as pretty flat because your values are not describing the forms. Remember, when the form turns away from the light it gets darker in a gradual way. The folds in the cloth almost look like rolling mountains. So what I did is do a drawing to show you some of the steps I use to rendering forms like this. I hope it helps :)
Hi @Avşin , I am definitely seeing form/structure in this drawing! Especially in some of the lower parts of the cloth, like in the lower left corner--you did a nice job showing that twisting 3D form. What I think really makes it look 3D for me is the reflected light you put in the shadows of the form. I think if you added more reflected light into the top of the cloth, where that big peak is, it would have more structure. Plus, I think moving that shadow higher and darkening the terminator (the darkest line of the shadow where the light meets the shadow) would add to the structure. And I think lightening parts of the cast shadow (that band of dark on the left side of the drawing) would make the big peak of cloth stand out. But other than that, I'm impressed by the beautiful structure you've got so far!