The button that opens the raid window is at the top of the screen. The button to start a raid is at the bottom of the window.

Wen the raid starts, family equal in strength will be matched for you. It will settle in your neighborhood on the world map. Note that the search for enemy family will continue even if you are offline.

When an enemy is found, the shields over your home camps will disappear and you'll be able to attack.

After the raid, a shield will appear above your camp for 96 hours. You will be able to start a new raid at any time.