From the Super Mario Wiki
The Elemental Tablets are three tablets in Super Paper Mario: the Stone Tablet, the Fire Tablet, and the Water Tablet. Mario, Princess Peach and Bowser find out how to get them after looking at the Cragvision sets throughout 5-2, the Gap of Crag. To get the Stone Tablet, Mario must run in a circle around the statue of Yoshi once to make it appear on the nose. Bowser has to light a fire under a stone archway to get the Fire Tablet. Last, to get the Water Tablet, the heroes have to dive to the bottom of the river in the first area to get it. Once all are collected, the heroes must climb the tall stone pillar near the Yellow Block to hit an invisible block, which extends a ladder to the tablet pillars. After inserting the Elemental Tablets into the pillars in order (stone, water, fire - this order can be seen inside the volcano in the background of the main area via a Warp Pipe), Cudge, a Pixl who allows the player to use a hammer, will appear and join the team.