The best way to measure the skinfolds is by using a caliper and if someone would assist you while you do your measurements.

Three Skinfold Measurement:

Shoulder Skinfold: Right under the shoulderblade
Trizeps Skinfold: Between the Shoulder and the Elbow on the Trizeps
Belly Skinfold: Approximatelly 2 centimeters beside the belly button vertically measured

T.G. Lohman: Skinfolds and body density and their relation to body fatness: a review. In: Human Biologie. Mai 1981, S. 181–255

Four Skinfold Measurement:

Shoulder Skinfold: Right under the shoulderblade
Chest Skinfold: Between the armpit and the nipple along the chest muscle
Armpit Skinfold: Right under the armpit vertically
Belly Skinfold: Approximatelly 2 centimeters beside the belly button vertically measured

National Health Center of America (NHCA)

###Seven Skinfold Method:
Shoulder Skinfold: Right under the shoulderblade
Chest Skinfold: Between the armpit and the nipple along the chest muscle
Armpit Skinfold: Right under the armpit vertically
Belly Skinfold: Approximatelly 2 centimeters beside the belly button vertically measured
Trizeps Skinfold: Between the Shoulder and the Elbow on the Trizeps
Waist Skinfold: In the middle between the last riba and the hip bode in a 30 degrees angle
Tigh Skinfold: 10 centimeters above the knee cap vertically measured

Jackson, Pollock: Generalized equations for predicting body density of men. In: British Journal of Nutrition. Nr. 40, Oktober 1978, S. 497–504
Jackson, Pollock, Ward: Generalized equations for predicting body density of women. In: Medicin and Science in Sports and Exercise. Nr. 12(3), 1980, S. 175–182