Adding or editing journal entries uses a calendar grid to pick the day you want. This grid contains "dots" which give a helpful visual indicator of how things are going:
If a dot doesn't have a colour, there is no entry so you're creating a new one when you tap on it.
If the dot has a colour, there is a previous entry you can edit (the colour indicates the mood for that day (red = bad, green = good).
If the dot contains a star, all the goals and habits available on that day were worked on (hooray).
When editing a journal entry, a few things to bear in mind:
For a new entry, the mood slider is set to amber (mid way) and the rest of the fields are blank. If you tap the save button without editing, this default blank entry won't save (to avoid clutter).
If you do accidentally create a journal entry you don't want, there is a trash can button in the top right of the screen. If you tap on that, you will get an "are you sure" message, and if you're confident in doing ahead tap "Delete".
When logging progress with goals and habits, be honest with yourself. Did you really make meaningful progress working on them?
Think about the good things that happened, even on a difficult day there's usually something to be grateful for and to learn from.
For things that were difficult, ensure you think about things to do differently in future. And think about next steps for these (which might go in your task list or calendar).
Don't overthink the mood slider, go with your gut.
Give the day a short and snappy headline which summarises it. When you search for journal entries, this will show up in the results, so a useful way to find things quickly.