We recommend using an acid-free permanent fine-tip pen to write in our photo books. Something like a Sharpie Ultra-fine tip pen. You can find these at big box online retailers and local craft stores. You may also use ultra-fine tipped permanent pens, as those will not smear (after they dry), but we always recommend using acid-free permanent pens on our books, as this will extend the lifespan of your book.
Feel free to reach out to customer support if you have any questions. Chat is available 24/7 or you can email us at hello@mixbook.com