Capture the starting point
Write down the plant name, pot size and window position. One clear photo helps when you return to the note later.
Keep observations separate
Record what happened before writing a possible cause. A yellowing leaf is an observation; lack of water is only one possible explanation.
Compare monthly
Look for the direction of new growth and the condition of the newest leaves. Slow improvement can be more meaningful than a dramatic change in one afternoon.
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.