Every one of our students at school keeps a field journal, so we're always looking for good mentor texts to share with them, to give them ideas from others who keep their own journals. The other part is to learn what to look for, why to capture it, what details to note in words and sketches? Of course those younger students do all of their notes in pictures, or by carefully copying down a few words. Slowly everyone improves their noticing. It's a great thing to practice, and then to know one is improving.