The civilian GPS (Global Positioning System) satellite system signal transmits with low power, line-of-sight at 1575.42 MHz in the UHF (Ultra High Frequency) band. The system is a constellation of 24 satellites launched by the U.S. Department of Defense. The system was made available to civilians in the 1980s. The satellites orbit the Earth twice a day and transmit to Earth. GPS receivers use triangulation to take the line-of-sight signals, from multiple satellites, and calculate the receiver's position. Blocking the signal can be accomplished by affecting the line-of-sight.
Drive a permanently mounted, GPS-equipped vehicle into an underground parking structure to quickly and simply block the GPS signal. The nature of the signals is that GPS is line-of-sight, and the concrete and the underground environment will block the signal.
Place a hand-held receiver in an environment that will block GPS signals. Placing it in a water-tight bag underwater will block the signal, as will carrying into a basement.
Test other marginal ways to block the signal. Usually GPS receivers will not get a signal inside a building, under dense foliage — such as in a forest. But sometimes they will. GPS receivers need to be able to see a minimum of three satellites for a latitude and longitude fix. They need four satellites for latitude, longitude and altitude calculations. So, you may find the lack of sky found in deep canyons or tall building will also hinder the reception of a full set of GPS signals.