Provided the surface where you are intending to lay the tiles is basically firm and even (not bumpy), then all you need to do is to clear the area or any debris, and lay the tiles directly over the top.

The only situations where additional surface preparation may be necessary is if the surface is either badly cracked or very uneven, or where you wish to lay the tiles over bare ground.

If you wish to lay the tiles over bare ground, firstly and most importantly you need to make sure that the surface is level (in other words not bumpy) and has a slight slope to allow for water to drain away quickly in one direction.

Then you need to remove all vegetation and then make sure that the surface is as firm as possible.  Ideally you should then lay a thin layer of fine gravel (but not sand) over the top of the prepared ground to improve drainage of surface water and make it easier to level the surface.

After that you should cover the area with a fabric type weed control barrier. You can purchase this material in rolls from e.g. Home Depot or Lowes. This not only prevents the growth of vegetation but helps to prevent the “feet” on the underside of the tiles from digging into the surface below.

When you have completed these steps, you just lay the tiles over the top of this prepared surface.