I find it helpful to use a mid value grey and a black copic marker. If you work with these you will be restricted to three values (third being the white of the paper). Use the broad tip to help avoid noodling and to focus on big shapes.

Hi, congrats for your first studies. They are the most difficult ones. Later you will get used to it and you will bocome bolder in your decisions. I think you did a nice job. It seems that you are really trying to find the main masses. As Ron said try to avoid shading, it distracts you from the exercise. Also, I would work more on the silhouettes shapes. Take your time to try make them read the best you can or make them interesting. You work your design skills that way. I hope it helps =D

Hey a while ago I also did a notan study of the first picture you shared. Maybe comparing them helps you, mine is of course also not perfect and everyone makes different decisions when doing notans. (I added 2 tones later, which is not needed at first). I think you are already making pretty good decisions and you are consistent with them. I would recommend to you to really just use 2 tones and to dont fall in the trap with pencil to shade / use more tones in the initial notan (mainly i see this in the first study). Also it think proportion can be pretty important in notans bc you can change the composition dramatically by changing the proportions of the pictures. (I see as a problem in your fourth study) Additionally I heavily recommend Sinix' video about value distillation which are pretty much notan studies:(https://www.youtube.com/watch?v=iwRa5qTnr8o&ab_channel=SinixDesign)