This question comes up frequently because it used to be available in the web version, but now the feature is disabled. Instagram itself doesn’t allow you to see who someone added last; everything is displayed jumbled. I tried various methods and realized that third-party solutions are essential. There’s an app called recently followed that helps track this and I’ve come to the conclusion that it’s more of a convenient analytics tool than just a curiosity, especially if you need to understand account growth or activity. Many people on forums write that it really saves time and gives at least some insight into what’s going on with their followings.