Don't vacuum nor brush (initially), first use gravity!
Take out the lens and be sure to fix the mirror (full battery so it doesn't get down, you know...). Then put the camera over your head, sensor looking down, and use a blower to blow as softly as you can just to make the grains fall by themselves (you may want to use protective glasses)
[If there are also free grains inside the camera, first remove them all before going for the sensor]
Close it and take a picture of a white/grey board and see if you can still see some spot there.
Repeat until you get tired.
If unsuccessful, then you will find more tolerable to pay a service to do mostly the same under a microscope.
Or, maybe, before that ask for some minutes of a binocular microscope in your friendly university or college around and THEN, sure of what you are aiming for, risk some hair of a brush touch a stucked grain of sand on the sensor.
If you dared to take the 5D to the beach (and change lenses there?) I'm sure you have the guts to clean it!