Edit and revise

Look at what you've got so far, and revise mercilessly. The arcanum's starting moves, its mysteries and the way they're unlocked, and its consequences should all reflect its concept and themes. They should suggest a story, but not dictate one. The pieces should all work together, tempting both player and PC to keep using the arcanum, despite the inevitable costs.

Write multiple drafts. Edit heavily. Be willing to discard ideas that you love. Share drafts with your players and other GMs. Be open to feedback. Get it as good as you can before putting it in play, but be willing to revise if something doesn't work.

Writing a major arcanum is a lot of work, but it can be very rewarding.