To visit the Vatican Museums, including the Sistine Chapel, you will need to purchase tickets in advance. Here's a step-by-step guide on how to book a Vatican Museums tour and tickets:

1. Visit the official Vatican Museums website: Start by going to the official website of the Vatican Museums at https://www.museivaticani.va/. The website provides detailed information about the museums, exhibitions, and ticket options.

2. Navigate to the ticket section: Look for the "Tickets" or "Visit" section on the website. You may find it in the main menu or on the homepage.

3. Choose your ticket type: Select the type of ticket you need based on your preferences. The options may include general admission tickets, skip-the-line tickets, guided tours, or special access tickets. Consider whether you want a guided tour or prefer to explore the museums independently.

4. Select the date and time: After choosing the ticket type, you will typically be prompted to select the date and time of your visit. Certain time slots may have limited availability, so it's advisable to book in advance, especially during peak tourist seasons.

5. Provide personal information: Enter the required personal information, such as your name, email address, and the number of tickets you wish to purchase. Make sure to double-check the information for accuracy.

6. Review and confirm your booking: Carefully review your ticket selection, date, and time before proceeding to the payment page. Some websites may require you to create an account or sign in to finalize your booking.

7. Make payment: Follow the instructions to make the payment for your tickets. The accepted payment methods may vary depending on the ticketing platform or website you are using.

8. Receive confirmation: After successfully completing the payment, you should receive a confirmation email with your ticket details, including any specific instructions or guidelines for your visit. Save this email or print out the tickets as you may be required to present them at the entrance.

9. Arrive on time: On the day of your visit, make sure to arrive at the Vatican Museums entrance on time. Allow sufficient time for security checks, as there may be queues, especially during busy periods.

It's important to note that ticket prices and availability may change, so it's recommended to check the official Vatican Museums website for the most up-to-date information. Additionally, there are various authorized ticket resellers and tour operators that can assist you in purchasing Vatican Museums tickets, but it's advisable to use reputable and trustworthy sources.