The notes on the levels are fairly easy to spot, and they're usually in a position where one will lead you to a few others.
If your struggling, follow this guide:
http://www.gamefaqs.com/console/xbox360/file/951686/3145**IMPORTANT: Do NOT attempt Bottles' jigsaw puzzles before collecting all 900 notes because some notes will be lost forever leaving the achievement unnattainable and requring the start of a new save.**