After you've connected to your Rest Mini, you'll be able to access all of its features in the Hatch Sleep App.
You can tap the play button on any sound to play it from your Rest Mini - after you've started to play, you'll see a player across the bottom of your screen that you can expand to control the Rest Mini's volume or set a timer.
The player will reflect what is currently playing on your Rest Mini, whether you turned on sound via your app or your Rest Mini itself.
You can tap the timer icon on the player page to set a timer.