To unlock the Elementary, My Dear Bayek trophy in Assassin's Creed Origins, you will need to complete one Papyrus Puzzles. There are 25 Papyrus Puzzles in the game (excluding DLC). To solve one of the Papyrus Puzzles, you will need to solve the clue to find the location and then loot them.
Each solved Papyrus Puzzle will reward you will 300 coins, 500 XP and a rare/legendary item.
You will have the opportunity to solve the first Papyrus Puzzle -- A Long Drink -- early in the game when you are in Siwa.
You can find a video guide containing the location below (credit to CamDeeGaming on YouTube):