Login and navigate to the page whose changes you want to monitor.
Click on the pencil tool icon at the upper-right and select View Page in CommonSpot, Work on this Page, or Work on this Page (All Changes).
Click the "Actions" button (in the content-spanning toolbar at the top) to display the menu, and then choose "Subscribe to Changes...".
Choose whether to be notified of any changes to the current page by E-mail or by the My CommonSpot display, or by both methods.
When you are done, click on the "Save" button.
Once you have subscribed, you can see a list of all the pages you are subscribed to, and from that list manage those subscriptions, in two ways:
Click on the "Tools" button (in the content-spanning toolbar at the top) to display the menu and choose "My Subscriptions." Unless the page you are looking for is in the top-level subsite, you will have to adjust the selection criteria and click on the "Filter" button.
Click on the "My CommonSpot" button (in the content-spanning toolbar at the top), examine the "Reminders" block on the right side, and click on "Subscriptions."