From: The assessment of physical fitness by correlation of vital capacity and and certain measurements of the body.  By Georges Dreyer and George Fulford Hanson, 1921, page 9.

“The subject should be seated on a high stool with his back straight, opposite the spirometer, the dial being so placed that he cannot see the readings.  This is done for the purpose of keeping him in ignorance of the readings while be examined, as it is found that any such knowledge tends to interfere with the accuracy of the results.  The neck, chest and abdomen must be free from any obstruction to free movement, such as collar belt or stays.  The subject is asked to fill the lungs to the maximum capacity, then the nose is held with one hand, the mouthpiece is placed well inside the lips with the other hand in such a manner as to prevent any escape of air round it.  He now blows steadily into the tube, and empties the lungs as completely as possible into the spirometer, being encouraged during the last period of expiration to make the utmost effort to expel all air from the lungs. The readings are given in liters and decimals of liters, to be read directly from the dial. After each expiration the needle on the single liter dial should be brought back to zero by the observer.  At the end of a long series of expirations the spirometer should be inverted, to allow any condensed moisture to escape. Five successive observations should be taken and recorded, the subject being allowed a short time for a short rest after each.”