Like most things in Earthdawn, journal entries are as much in-character as out-of-character. In-character, the stories are collected by the Delver's Athanaeum, which pays a small amount of silver for true tales of the post-Scourge world. These are archived in Travar, with copies periodically sent to the Great Library in Throal. The legend point reward reflects how your journal builds your legend, as people come across your stories.
It is not in-character for everyone to produce the written word. Feel free to assume that a librarian is available to take dictation of your oratory, or to present your own legend in script format, or as images. (Can't draw? Maybe descriptions of the paintings your character submits are more your style.)
A good journal entry should do two things:
1. It should tell the story of the adventure, so that someone who was not on that mission has a reasonable grasp of what happened.
2. It should convey your character's voice, attitude, and the way they see the world. Someone reading your entries should be able to guess your Way and your personality traits with reasonable accuracy. If you find that this is repeatedly not the case, you may wish to reconsider whether the personality traits you have chosen accurately reflect the character you are roleplaying.
If it achieves both these things, a journal entry is worth 10% of the mission's legend point reward in silver, and 5% in additional legend points. For example, the journal entry for a mission that was awarded 500 legend points would be worth 50 silver and 25 additional legend points.