Take a large plant pot (5 gallons or more, depending on the size of your umbrella) and place it next to the area where shade is desired.
Place the bottom of the umbrella in the pot, and tilt it to the desired angle.
Fill the pot with sand until the umbrella is able to remain in the desired position without you holding on to it. The weight of the sand and the plant pot should hold the umbrella in place.
Adjust the angle of the umbrella by shifting the bottom within the plant pot.