To access the Battles page, the first step is to click on Battles in the left side menu:

You will be presented with the Battles page. Press Click to start a new Battle:

A screen will open where you can pick your opponent.

Once you select your opponent you will need to select the metric to battle on.

Once the metric is selected, you will click on the arrow on the right side of the top box to set your goal and dates. A goal is not required.

Once you've decided on the dates and/or a goal you will click on the arrow to the right again to decide on how many coins you will bet on and your personal and screen message to your opponent.

You can also decide to make this a private battle by checking that box.

Once this information is filled in hit "Send" and your opponent will receive the invite.

Once they accept, you will receive a notification and the battle is on! If they decline, you will also receive a notification.