First make sure you are signed in to Shared.Gallery:
You must be the event owner to do this.
Login to the Booth.Events dashboard
Select "Sign in to Shared.Gallery" from the sidebar (dropdown on mobile)
Wait until you see that you are signed in to Shared.Gallery. You'll be signed-in for this browser in *all* galleries.
How to download the ZIP
Navigate to the desired gallery.
Click on Menu.
Click Download All
Click create Zip
The Zip will generate and then you can download it by clicking or taping on the name of the Zip file.
Depending on the size of the Gallery it may split into multiple Zip files.
If the Zip is created before photos are deleted from the gallery they will be included in the Zip going forward.
I want my guests to be able to download the ZIP file too. How?
By default only the event owner can download the ZIP file, but you can change this so that some (or all) guests can download the ZIP file too. To enable this please see this article.