You comb it to one side, snip. Comb it over to the other side, and snip. Comb it straight up from the neck and snip. It's kind of like hairdressers cut your hair -- they hold it in their fingers and cut it while it's bent this way and that way around their fingers, so that it doesn't end up blunt and straight. It has kind of feathered, neat edges.
Another way to get that look is to cut into the end of the hair, not across the end. You would get it to the right length and then use very sharp scissors to cut the very ends of a few hairs, or use thinning shears to cut up into the ends of the mane hairs. In other words, when the mane hair is pointing straight down, the scissors would be pointing straight up, just at the very ends of the mane.
I've done this before, especially with the thinning shears (it's faster) and it maybe wasn't show perfect but it looked really good to me. A friend of mine rides dressage and that's what she does, too, instead of pulling.