Instructions
Craft the ancient stele after you collect all nine hieroglyphics pieces. A message will pop up after you collect the final one informing you that the Ancient Stele plan is available.
Walk over to your crafting bench until it is highlighted with a circle around it. Press the "A" button to view the available crafting plans.
Place your cursor on the icon of the picture that represents leisure plans. Press the "A" button to view the leisure plans you can make.
Locate the "Ancient Stele" plan within the available leisure plans using the directional buttons. Click on the plan to make the stele at your craft bench.
Inspect the "Ancient Door" located at Geyser Plains. The door will automatically unlock as long as you have the ancient stele in your inventory.