`how do i us rmy lasso

First equip the Lasso, then aim at your target, finally press the trigger. The Lasso will automatically tie your target up.

Axionas got it right.
I always find shooting them in the leg helps, or lassoing them after they fall off a horse.

You get the lasso in a story mission for Bonnie McFarlane

To use the lasso, you equip your lasso, press L2 to target the opponent then press R2 to lasso them. You have to be close enough for the lasso to reach though. Then stop the horse with R1, then press the triangle button to dismount while still holding on to L2, go over to the opponent and hogtie them with the button represented on the screen. Pick them up by pressing the button represented, put them on the horse by pressing triangle, and your on your way to a wonderful bounty award.

Same as any other gun, target and fire the lasso. If they're on the ground like after a bullet in the leg, that would help. Run over to them and press Triangle to hogtie.