As leaderboards and achievements used to be discussed together as one topic, Epic has done their best to make the Achievements process similar to the Leaderboard process, which is as follows:
Use the Write Achievement Progress node.
Assign a value for Achievement Name, which represents the ID in iTunes Connect.
Pass a Progress value to it if it is a progression achievement.
You don't have to use a user tag, but this value will be returned anyway on success and on failure.
You can also use a similar node on the leaderboards to display the Game Center achievements screen by calling Show Platform Specific Achievements Screen. This too is named Specific as it is used with Android to show the Google Play Achievements screen.
As leaderboards have the functionality to download the progress and get its values, achievements have similar functionality too. However, as an achievement has a progress and description and not just progress (unlike the leaderboard), achievements have four...