This FAQ currently only explains how to create memories if you have message boards enabled and a comments link for the entry you want to add as a memory. It should be edited to include a method that will also work for people who do not. I brought this up in Suggestions and am moving it to its proper forum.
This method should work for any user.
go to your calendar view click on the subjects link next to the month containing the entry select the entry you want to add to your memories Use the heart icon to add the entry as a memory