What to record
Note the date, plant, location, soil feel, and anything that changed. Two lines are usually enough.
What not to change at once
If a leaf yellows, avoid simultaneously moving the plant, repotting it and adding fertilizer. Small, separate changes are easier to understand.
The monthly review
Compare new growth and general leaf condition with your earlier notes. Healthy change is usually gradual.
Include the weather
A long stretch of hot or cloudy days may explain changes that otherwise look mysterious. Add this context to the journal.