firsly, set your WB to auto. find a patch of grasses or foliage then focus your lens on the grasses. then put your IR filter on. your camera must be set to MANUAL. then take a shot of the grasses (shutter speed depends on the lighting condition, the best time is from 10am-3pm when there are plenty of infrared rays. in my case (canon 550D) i set my ISO to 100 and expose the sensor for approximately 60-80 seconds (the lighting condition is during noontime). after shooting, see what you got. your photo should look red and white. this will be the time you need to set your WB to custom WB choosing the grasses photo. once you've set to custom balance, you're now ready to take IRs using Custom WB.
your photo must look like this on the camera LCD after setting your WB to custom. as you can see, it is no more red and white. the foliage is bluish white and the sky is a bit orange/brownish.
Same photo after tweaking it using the Channel mixer.