I have created kindle collections on my iPad. How do I synchronise them across devices?
Make sure sync is switched on in settings (it is by default) Hit sync. Job done.
open other device (assuming it supports cloud collections) and hit sync. Job done.
To add a little to BlueOfTheNght's post, I find that any newly created Collection will not appear on other Kindle devices promptly. I usually have to wait overnight.
and you'll need to Add them to Downloaded in order to see them under the Downloaded category.