Use a format you will return to
A pocket notebook or a simple file on your phone can work. Keep the structure short: date, place, route, conditions and one thing noticed. Avoid collecting so much information that writing it becomes more work than the outing itself.
Separate facts from impressions
You might record a measured distance beside the impression that a section felt steep. Both are useful, but they answer different questions. Label uncertain observations instead of turning a guess into a confident identification.
Include practical lessons
Note a transport connection, a water source you still need to verify, or an item that was uncomfortable to carry. Recheck information that can change before relying on it next time. A personal note is a prompt for planning, not an official conditions report.
Leave room for a drawing
A rough sketch of a leaf, a view or the shape of a path can help you remember something that a list would miss. No artistic skill is required. The notebook belongs to your experience, not to an audience.