Sakura's Senpai Monitoring for Dummies:
Step 1 - Quietly (stalk) follow the Senpai.
Step 2 - Watch for any disturbances in the daily routine of your Senpai. Any interactions with the opposite sex should be (sabotaged) channelled towards a more satisfactory outcome.
Step 3 - Watch out for sisters or other relatives. Such (underhanded witches) dangerous individuals are the most threatening to your efforts.