To start a Challenge against a teammate, enter the coalition chat and click the "Challenge" button in the lower-left corner. Then, tap the “Create” button, as you can see in the images below:

After creating a challenge, you must define the battle type: Legacy or Standard and choose your deck.

If you need to cancel the Challenge, just click the "Cancel Challenge" button.

If your teammate challenges you, you need to click the "Challenge" button to see the created battle. In the "Open challenges" screen click the "battle" button to accept the challenge. Keep in mind that multiple players can create a challenge at the same time, so always check the player's username and deck restriction type.

After accepting a challenge, the deck selection screen will appear, select a deck of your choice that matches the battle restriction, then just click "Confirm" to join the battle.

Once your opponent's turn ends, click to continue.

Every time your opponent opens his hand or his Planeswalker, a yellow line will appear to you to indicate this action. Remember that your actions will also be shown to your opponent.

When a message arrives in your coalition chat and you are in a match it will appear as the image below.

You can also open coalition chat and send messages during the match.

The result of the battle appears in the coalition chat, as shown in the image below.