How do I add mementos and notes?

[Wevah]

I'm trying to add a note to my game and right afterward to add a memento to the journal. I don't know how to do this though, the wiki is a bit confusing.. I mean, it seems I can only add journal entries if I have my custom story setup, is this true? (I only have one level as of now).

AFAIK, yes. The texts for the journals are kept in a localized file separate from the maps themselves. (Setting up a custom story should be about 5-10 minutes of work, though.)