Find a good location for your flagpole, which should be located at least 15 feet away from any overhead wires.
Dig a hole using your posthole digger. The hole needs to be as deep as the flagpole sleeve is long, plus an additional 2 inches. Make the hole twice as wide as the flagpole sleeve. When the hole is done, use your shovel to make the bottom of the hole an inch or two wider than the top.
Place 2 inches of gravel into the bottom of the hole and tamp it down with the end of a 2-by-4 board.
Place the sleeve into the hole. Place a level across the top of the sleeve, then along the inside of the sleeve. Make adjustments on the sleeve positioning until it is perfectly level and plumb.
Mix together the concrete and water according to directions on the concrete bag. Place a small piece of plastic over the hole at the top of the sleeve and tape it in place. Pour the concrete around the hole until it is about two-thirds full. Check the sleeve to see if it is level and make any necessary adjustments. Remove the plastic and carefully fill the rest of the hole with concrete. Use your trowel to angle the concrete near the lip of the sleeve away from the sleeve, so water won't pool against the pole.
Wait about 24 hours for the concrete to set. If your flagpole has fittings that need to be attached, you can attach them now.
Move the flagpole to the area near the hole. If the pole has a flash collar that sits around the base of the pole, slip it over the top of the pole now and let it rest there. It will slide into position when you raise the pole.
Coat the bottom of the pole in Vaseline. Only coat the area that will be in the sleeve, so you can easily remove the pole in the future.
Place the bottom of the pole into the hole and tilt the flagpole upright. Once the pole slides fully into the sleeve, press the flash collar all the way down to the ground.