How do I mark a medication as taken?

Tap "Overview", then the "+" on the "Log a symptom or a reading" card, and tap "Meds". If no medications are listed yet, tap "Add manually" (or "Connect records") first; then select the medications you take, tap "Next", choose "Track" (or "Track + Reminders" to also get reminders), and tap "Save". After that, "Meds" opens your daily list, where you tap a dose to mark it taken.