I started doing Dual N-Back from level 2. It was hard. My D2B rate was about 50% those days. In a couple of months of constant trainings I reached the 7-th level. That may seem great for beginners and not a big deal for some advanced n-backers. Doing that exercises I acquired some good training experience. I also read about how N-Back should and should not be done. So, here I gathered some advices for you.