Here is a simple and useful carpet design that I’ve used in several models, though most recently my Modern Living Room.
For this example you start with a 6×8 plate, and then tile the center 4×6 area. Two layers of plate plus a tile around the outside make the wood floor, and the carpet itself is made of grill bricks. Of course you could substitute other one wide bricks if you like.
Since one brick layer on its side is 2.5 plates high, you get a nice half plate offset to simulate the depth of the carpet lying on top of the floor.
Here you can see the technique used in a full scene.
Enjoy and let us know what other designs you would like tutorials for!