Select any floor covering. Drag the flooring directly on the ground to cover the entire area where your structure will be built plus a few extra tiles on all sides. If you need to stop and restart, begin over an already placed "ground tile" - not on a bare piece of ground. This will level out the minor imperfections in the ground and keep them leveled throughout the building process! Unless you have other plans for your lot it's usually best to start from the sidewalk and work back so your house will be at sidewalk level rather than above or below it

If you want to build on raised terrain, raise your ground as desired and level with the leveling tool. Do NOT depend on this tool to make your ground completely level! Instead, proceed as above on the raised portion of your lot.

Now, build your structure right on top of these "ground tiles" just as your normally would. If you're using more advanced methods of CFE that require raising/lowering terrain and/or adding foundations and walls outside of your structure, do so right on top of the "ground tiles" - remember, if you need to extend the tiles you must start on top of an existing portion.

To remove the tiles outside of the structure, select any flooring and hold down the Ctrl and Shift keys and click on one of the tiles. Try this same method for the tiles under the foundation - if it doesn't work (a problem I've had), drag across the tiles with the sledge hammer tool.