Requesting reinforcements is easy - simply tap the Alliance Camp situated just below your Castle in your city, then tap the Request button.  

Enter your request message into the text box – it’s a good idea to specify exactly what troops you want so your alliance members know what to send you. Remember that the text is moderated and any profanities won't be registered.

Once sent, your request will appear in your Alliance chat. Your alliance members will be able to see the request and donate troops to your cause. The number of troops you can receive will change depending on the level of your Alliance Camp.

Once you've received your reinforcements, just look for a land to attack, choose to "raid" or capture" the land and tap on the reinforcement during battle prep to include the ones that was sent you. During the battle, summon the reinforcements by tapping the summon button in the bottom right corner.  The reinforcements will deploy automatically and join you in battle.

Remember that reinforcements are temporary units, who have been sent to help you for just one battle. After they’ve fought they'll leave your army, regardless of whether they survived or not.

Players can only request reinforcement every few hours that the option refreshes. You can make a request again without waiting for the refresh time to lapse by paying some Gems.