If preparing a mane for plaiting, you need about 6 inches of mane at the same thickness throughout. I rarely find I need o thin a mane, but if its necessary I would use a Smart Manes. I do this by brushing the mane onto the off side, then lightly comb, one stroke at a time, down the mane, until the desired density.
I find the quickest and most effective way to shorten a mane is the cut and comb method. Starting near the withers, comb through the hair, back comb the top layers of mane, then cut underneath, holding the scissors at an angle. Comb down every time you cut so the hair falls in a slightly different place and you’ll get an even but natural look.
The mane should lie on the offside (right), but occasionally you find horses mane that naturally lies on the opposite side. Don’t pull or shape it so it continues to do so, encourage the mane to lay on the right side by ‘laying’ the mane with a damp sponge and/or using an appropriate stretch hood or neck cover.