Before you start, first make sure your device has enough charge and that it is compatible with Apple's ARKit 2 framework.
Next, find a great space to play in. For the best results, make sure your room is well lit and that you have enough space to move your device freely - there's lots to explore in Little Red. Make sure you avoid pointing your device at direct sunlight too.
Now, open the app by clicking on its icon. Then follow the simple instructions it gives you.
Finally, find a flat surface, such as a table top or a space on the floor. If the surface is smooth or has few visual features - as you'd find with a white table top or a plain carpet - you might want to place some small objects in the playing area. This will help the app track the surface properly.