If you take hold of the horn with your hand, you will feel that the end closer to the sheep's head is warm, whereas the tip is cool. The warm part is where there is a blood supply - it's an artery so will bleed profusely. Make any cut above that point, so you are cutting only into horn beyond the blood supply. Don't forget to protect her eye from horn dust.
If the horn is not too thick, you can also use loppers or bolt croppers.