You can join an alliance by the following steps.

1. Tap [Alliance] at the bottom menu.
2. Tap [Join] on the "Stand Together" pop-up.
3. [Apply] to any available alliance.
4. Wait for approval if the alliance requires applications.

You can also create your alliance by consuming 200 diamonds.

1. Tap [Alliance] at the bottom menu.
2. Tap [Create] on the "Stand Together" pop-up.
3. Customize the alliance information
4. Tap [Confirm]