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.
Photographs help
Take a quick picture from the same angle each month. Side-by-side images can reveal gradual growth that is easy to miss daily.