I use 1" round white stickers from the dollar store. I use black sharpie to label row A1, A2, etc for the first row. b1, b2. Etc for second, and so on. If I change my mind about block positions, the stickers are easy to peel off and switch. Much easier than pinning labels on, much more dependable than post-its that just have a narrow strip of adhesive, and the tabs are cheap! I take a picture after each arrangement I play around with.