To book a purchase in Tally Prime, you will need to follow these steps:

1. Start Tally Prime and open the company for which you want to book the purchase.
2. Go to the Gateway of Tally and select the Accounting Vouchers option.
3. In the Accounting Vouchers menu, select the Purchase option. This will open the Purchase voucher screen.
4. In the Purchase voucher screen, enter the date of the purchase and the name of the supplier or vendor.
5. In the Items section, enter the details of the items or services that have been purchased. This should include the item name, quantity, rate, and amount.
6. If applicable, you can also enter any discounts or additional charges in the Discount and Additional Cost sections.
7. Once you have entered all the necessary details, press the Enter button to save the purchase voucher.
8. The purchase will be booked and recorded in Tally Prime. You can view the purchase details by going to the Display or Alter option in the Accounting Vouchers menu.