How do I add someone as a Friend?

To add someone as a Friend, or Follow someone, visit the user's profile and press this button once:

If the other user has already added you as a Friend before, you will be able to find this user on your Friends page.
Otherwise, the user may be found on your "Following" page.
You can find these pages on your profile.

See more information - What is the difference between Friends, Following and Followers?