Once you have downloaded the game onto your device and have opened it please go through the tutorial. You can tap through each step quickly. At this time you will be signed on to a guest account.
From there simply tap the Player Name in the top left > tap on the Gear Icon in the top right > tap Gaea ID > then enter your login credentials.
At this point, if you still see a new city, you’ve been put into a new world and need to log into the previous world where your city was located. To do this tap your Player Name again > tap the World tab > then locate your previous world and tap Return.
You should now have access to your account on your new device.