The video tutorial of this topic can be found here.
If you have an ARKit device (usually iPhone 6S and up), you will be prompted to explore your surroundings.
Here are a few tips to get started:
Aiming your device's camera at the floor or table you would like to place your 3D model
Slowly start scanning the surface with your camera
Tap on the screen to place your 3D model ONLY when a green square appears
Difficulty finding a surface? Green square not showing?
It may be too dark for your camera to detect surfaces. Turn on the lights in your environment or tap on the 'Flashlight' icon.
The camera may be having difficulty because your surface is too monotonous and does not have enough visual markers. This may occur on patterned surfaces. Fix this by adding some items like books, magazines, pens, chairs to the surface.
You may be moving your device to quickly. Slow down and rescan your surface.