1. Open the game and click on the pet you want to clean the pen for.
2. In the pet's profile, click on the "Hygiene" tab.
3. Click on the "Clean Pen" button.
4. Your character will pick up cleaning tools or supplies and a prompt saying 'Start Cleaning' appears for a split second before the pen turns visibly cleaner for several seconds and eventually looks immaculate, while the hygiene progress gauge on the right fills.