Turn off two-step verification on your Apple ID before beginning the process (if applicable).
Open Smart Switch on your new Samsung device, then tap 'Start' and read the terms of service, then tap "Agree".
On newer Samsung devices, you will find Smart Switch at Settings > Cloud and accounts > Smart Switch . On older ones, you may need to download it from the Play Store
Select "Wireless", then "Receive", then "iOS".
Enter your iCloud username and password, then tap "Sign-in".
Deselect any information you don't want to copy, then select "Import". Once you've imported the basic information, you will be given the option to download media content such as photos and documents. Untick anything you don't want to transfer, then tap "Import".
Once the transfer is complete, tap "Close app". Don't forget to also disable iMessage on your iOS device.