On or within each video, Summit series, Category or Topic you will see a plus sign.  You can click this to save the item to your Watch List.

Once added, it will show as a check mark.   And is accessible under My Watchlist from the menu at the top of the 365 page.

To remove the item from your Watchlist, simply click on the check mark and it will turn back to a plus sign for adding it to your Watchlist.