That is something you need to see done by some one that know what they are doing.If not it can be the biggest mess you ever saw.You backcomb with a comb and pull the long hairs.Go from poll to withers and it takes awhile go slow look it over.See if can find some one close that shows horses or fits horses for sales.
A quick, easy, and fairly good looking way to pull is to get a thin toothed metal comb. Run the comb through the mane, stop at the desired length, twist the comb once, then pull. Along with clampitt, i also agree the best way to learn is to watch someone who knows what they're doing. Best of luck!