What it's for
A working record, not a scrapbook of aesthetics. Its value is entirely in being able to look back and see what you actually did, rather than what you meant to do or now remember doing.
What to write down
The date. What you did. Tools or ingredients, where relevant. Where the practice came from, whether that's a book, a website, a friend or your own head. What happened, and what you noticed at the time. What you'd change next time. That's six lines, and it beats a beautiful page you never re-read.
Note observations, not verdicts
Write 'slept better, though I also went to bed earlier' rather than 'it worked'. Keeping the experience and your explanation of it separate is a habit worth building from the first entry.
Cite your sources to yourself
Writing down where a practice came from takes five seconds and saves an argument with yourself later, particularly when someone asks whether it's traditional.
Private by default
In this app, your Book of Shadows lives on this device. If you switch account backup on in Settings, a private copy is kept with your account so another device can pick it up. Either way it stays yours: it is never published, and other users cannot see it.