What helped me a lot was bringing a little journal to jot down notes while shadowing about the procedures being done, interesting things that happened, what I learned, etc. Then when I got home I would write it into a word doc where I compiled all of my shadowing notes and hours. It'll be useful to have those notes when you fill out your application because there's a description box you have to fill in for the shadowing hours you complete. Also, it helps to jot those down so you can talk about it later in an interview or write about it in your PS.
One more thing, probably most dentists won't care that you're jotting down notes but I made sure just to quickly ask if it would be alright to bring a little notebook with me into the procedure room. It doesn't hurt to ask!