Here's what I have heard of through other people
Brush and comb through to remove all the tangles. Dampen the mane and comb the flaxen hair down and out of the way. Trim the black hair first, don't cut it too short so if you mess up it'll grow out faster! Trim the black into a neat curve to emphasise the crest.
The do one side of the flaxen hair, then the other. Follow the top line of the black hair. Remember that although you'll aim to get both flaxen sides even, you'll only really see the flaxen hair from one side or the other, so its not vital if they don't exactly match (unless you're showing!)
I'd use sharp hairdressing scissors and a wide toothed comb but other people prefer clippers.
Don't worry about getting it perfect first time, just keep practising.
clipped even or with 1/4 to 1/2 inch of black stripe standing above the white crest.