Signing up for Keeper.sh now leads into a guided setup that ends with your events copied. It has three steps: connect a calendar, add the calendar where your busy time should show, and start syncing.
Before you start, Keeper.sh says in plain words what the sync will do, for example that your work calendar will show you as busy whenever your personal calendar has an event. Once you start, it reads your calendars at once and shows the copying as it happens, with an estimate of the time left. When it finishes, you see the events that were copied, and you can add a test event to watch one arrive.
The setup also handles the problems people hit most when connecting a calendar. A work or school Microsoft account that only allows apps its IT admin has approved gets an approval link you can copy or email to your admin, and the setup carries on once they accept. iCloud and Fastmail say when they need an app password instead of your account password, and point you to where to generate one. A calendar link that does not work says why, and one that works is named after the calendar it carries.
Each other connection failure now gets its own message instead of a general error, and the dashboard holds back its upgrade offer until you have a sync running.
Linked accounts and calendars covers connecting each kind of calendar.