If you have donated to the 'Butterfly Meadow' you do not need to pre-book your visit to view the installation. 

However, if you do wish to pre-book you can do so on the Sandringham website.  You just need to enter the code Butterfly2 at the checkout for complimentary access to the gardens for yourself and a guest. You will need to bring proof of your donation on the day of your visit. The Gardens will remain closed from 23-31 July inclusive.