Open the Mindfulness app -> State of Mind -> Get Started and choose the Log how you've felt overall today. By default, Neutral is selected. You can use the Digital Crown to browse through different available options and tap the Checkmark once you've made the selection. Then, select the option(s) that best describes this feeling and tap the Checkmark. Finally, choose the factors that have the biggest impact on you and tap the Checkmark to log and track your mood on your Apple Watch.