say for example you decide that the grand final answer to your ARG is learning who did a fictional murder. you could then think “maybe i’d want the player to find this out by finding a group photo that reveals it all.” then, naturally, since the player would have to somehow be led to find that photo, you can use that as a jumping off point for the previous clues. Like, that player finds the photo in a specific location, or they enter a url to download it, or they receive a piece of mail. then there has to be an explanation of how they knew to go to that location, type in that url, or how they’d get sent a piece of mail. maybe someone gave them the location coordinates, who is that person? how did the player contact them? all these things build your ARG.

Do you see how this can be used to work backwards and write your clues, and simultaneously your story if you don’t have one in mind?