In classical Feng Shui, the concepts of direction and location are two of the most important factors to consider. An accurate measurement of direction must be obtained before any system of Feng Shui can be undertaken.
You will need a reliable compass. A Luo Pan is convenient because it has the 24 Mountains marked on it, but an ordinary compass will suffice. No matter which you choose, be sure to use a compass that is accurate and easy to read.
The two directions commonly obtained with a compass are the House Facing direction and the Door Facing direction. Of these, House Facing is the more important.
In general, a house is built with an architectural frontage, a side that faces a landscape feature like a main road, a lake, a golf course, etc. The House Facing is the same as this frontage. More specifically, the Facing of a house refers to the side that is most Yang in nature. The main door may or may not occupy this side of the house, although it usually does.
In apartments, lofts or condominiums, the Facing of a unit is determined by the Facing of the entire building. Again, most of the time this matches the building's facade. The main entrance or lobby may or may not match this direction, so it should not be considered as definitive evidence of the building's Facing. If the building is round, the main entrance is usually the same as the Facing. If the building is not round, but has no obvious facade, consider the side of the building that has the most Yang energy (i.e. faces the busiest street and/or receives the most sunlight).
Only if it is difficult to determine the orientation of the building and it has more than one side that seems equally Yang should you use the main door direction as the Facing of the building.
Once you have determined which side of the house or building makes up the Facing side, stand outside the structure at a distance of one or two feet, in the middle of the Facing side, with your back to the house and parallel to the wall. Be sure to remove jewelry from your hands, since anything metal might interfere with the compass. Hold the compass level at approximately waist level and let the needle settle down. Orient the North marking of the compass with the needle and read the degree (or 24 Mountain marking on a Luo Pan) that points directly away from you. If the needle will not settle down, there is probably magnetic interference. Take one or two steps forward or back and try again.
Once you have obtained the Facing degree measurement at the middle of the facade, take a few more readings at different locations and maybe slightly different distances from the structure. Do this until you feel that the measurement you have taken is accurate.