Just over a week after Niantic revealed that Buddy Pokemon would be making their way to Pokemon Go, the update has finally begun to roll out for all iOS and Android users. Niantic actually said that the app was "in the process of being updated" over the weekend, but the update didn't appear on the App Store and Google Play for a vast majority of players until Tuesday afternoon.
Once you've installed the update, you'll need to follow these steps to choose a Buddy Pokemon to hang out with your trainer:
Tap the trainer's face in the bottom left corner of the screen
Tap the menu button at the bottom right corner beneath your trainer
Tap "Buddy," find your favorite Pokemon and tap on it
You should now see a Pokemon next to your trainer on the trainer screen as well as next to your trainer's face on the map screen. Remember, you can get a Pikachu to sit on your shoulder if you walk with it for 10km, so you might want to make that your first mission. Or you can just choose the biggest Pokemon in your collection.