There's 2 ways to add friends, for this you need to go to the lighthouse in the bottom corner of the map, or press the symbol with 3 exclamation marks on the right side of your screen:
Add their referral code (Mine is for example 392e). Note: You can only add the code from one person at the time of writing this.
The second way of adding friends is by using Facebook. Connect the game to Facebook, and either invite friends to play the game, or add persons that already have the game( there's a list with people who also want friends with the game over here: Facebook friends for daily presents - Add me topic
Note: It might take some time before Facebook friends show up on your in-game friends list.
After you've added at least 1 friend, you can start sending them gifts(food, gold or energy) and they can send gifts back. You can only do so once every 24 hours, so depending on how many friends you have, you can earn this achievement in 1 to 15 days.
The achievement didn't pop for me, after I gifted gifts 15 times. It popped when I started the game again, a few hours later.
Feel free to add me on Facebook, or post your own Facebook profile down below.https://www.facebook.com/profile.php?id=100009908161959
Note: progress does carry over from WP, even unlocked achievements unlock when you start the Windows 8 version, and choose to use your existing village.