babs, it is quite easy. lay the BH down flat on a stone or turn two blades up flat and stroke with a file (my preferred method).
When using a file, it helps to fill the file with chalk board chalk first so the filings don't embed in the grooves of the file impeding progress.
If they are quite dull when I start, I make 5-7 passes with moderate or more pressure before rotating sharpening one side of two different blades each time. I continue until I have fully rotated the head thus hitting each side of each blade.
Now do it again using one less pass each rotation with decreased pressure until you get to one pass per rotation. At this time you should be using so little pressure that you are barely touching the head. I will continue to rotate until I'm satisfied with sharpness.
If desired, you can further polish the edge with a fine stone and then stropping but I haven't found this to be necessary.
thanks, bigjim