First follow the instruction on Apple website to login your iCloud account.
Open Keys on you iOS device.
For first time user, Keys will show the Setup iCloud view.
Tap the iCloud switch to enable iCloud Sync. iCloud Sync is an In-App-Purchase item, if you haven't purchased it, you will be asked to make the purchase first.
After you enable the iCloud Sync, Keys will check if there exists password database file in the iCloud since you may have already synced data with iCloud from your Mac before.
If Keys is able to find your database file in the iCloud:
Keys will just use that file. Keys will then skip the password setup window and present you with the login window:
After you login, you will see a cloud icon on the top middle of the screen, this cloud icon appears if you enable iCloud Sync:
You are all set. After this point, your data will be synced automatically in the background.
Sometimes, even if you have database file in the iCloud. Keys may still not be able to find that file temporarily since it takes time for iCloud to transfer data to local disk. In this case, Keys will think there is no database file in iCloud and will ask you to create a new master password. Don't panic, you can just go ahead and create the same master password as the one stored in iCloud. When iCloud finishes syncing the database file to local disk. Keys will automatically merge your iCloud data with your local data.
If you actually don't have database file in the iCloud:
You should tap next button in the Setup iCloud view. After you create your master password and unlock the database, Keys will ask you to upload the file to iCloud shortly after:
Since you have never uploaded data to iCloud, tap 'Upload data to iCloud'. After this, you are all set, Keys will sync data automatically in the background afterwards.