On your new device, you would select Settings/ General/ Reset/ Erase All Content and Settings, which will erase it, then follow the prompts to log into your iCloud account and select the backup from your old device. Note that since your new device will be erased in this process, anything new on that device which is not in your old backup will be lost.
If you did not select your iCloud backup when you first set up your device, simply logging into your iCloud account does not load the backup, but the process above will.