Nice start on these. You are headed in the right direction. When you get your shapes worked out well, pay very close attention to the values. You want to match the values you see as closely as you can. It is important to be very honest about what you are seeing. try to put the accurate value down with each stroke as otherwise you end up having to fix things along the way and being accurate will save you time. Really take the time to observe and compare and choose the right value. If you are off, adjust it, don't keep working and come back to it. You are doing great...just need to focus in on value a little more.
Nice studies. Be sure to use the same background value on the most recent study. As for the others you are almost there on your values and if you really focus on value matching and getting your values as close as you can...flip the images and even back away across the room and look at them from four meters away, you will see the differences and can make big improvements very quickly. That is the next step for you. You should be able to get your values fully under control in the next few images. Just be sure you are double checking and triple checking.
You're doing pretty awesomely-
Looking at your first three studies I think you could use a harder brush so your studies don't look so blurry- you'd have to spend more time blending but I think it would still give your work a thicker feel to it. I think i remember reading somewhere that painting with a harder brush also gives you more confidence in your brush strokes? I dunno if that's true but its still wortha try-