If you are familiar with the bagua, or the feng shui energy map of your house, you know that there are eight feng shui directions that a house can face.
To find out the sitting direction, you need to take the compass reading of the back door of your house. If your back door is to the side of the house, then you can take the reading of the feng shui direction by standing parallel to the back wall of your house.
However, the feng shui house sitting direction is rarely used, as it does not give more info than the house facing direction.
The most common feng shui way is to take the compass reading of your front door, which will help define your house facing direction, and from there define the bagua, or the feng shui energy map of your house.
If you have a question about feng shui, please ask your question at the feng shui forum.