We have explained how you can do this in the guide below.
How To Make A Pokemon Your Buddy In Pokemon GO?
It is quite simple to assign a Buddy in the game. Just follow the steps given below and you will have a new Buddy in no time.
Open the Pokemon Go App. Click on your character’s profile located in the bottom left of the screen. Select the Choose Buddy option. Now, you will see a list of all the Pokemon you have caught so far. Select the one that you want to choose as your Buddy. It is quite simple to swap buddies and you can change as many as 20 buddies a day so picking one is not a binding option. Confirm that you want the selected Pokemon as your Buddy. The Pokemon you have selected will now appear with you in the game. Depending on the size and type of the Pokemon, they can be behind the trainer, in their arms, flying overhead, or behind them. You can now collect Candies for doing tasks like walking around with your Buddy.
Once a Pokemon has been assigned as your Buddy, they cannot be assigned to a Gym nor transferred. This is all about how to make a Pokemon your Buddy in the game. We hope that the guide was helpful for you. For more Pokemon Go guides, check out How To Make Pokemon Stronger In Pokemon GO?