End bars installation

i'd just carefully cut the grip off the outside, put the bar ends on and tighten them up.

a couple of hints, don't point them up too much, about a 20-30 degree angle is more than enough, you will find that when you move forward to use them that having them sicking up will be a pain in the butt

yes they are worth it especially for XC riding, on steep climbs you can grab them, stand up and lean forward a bit to keep the front wheel down without feeling like you are going to go over the front

Ive just fitted some to my bike and there ok but you have to nudge everthing towards the center of your handlebars eg levers shifters to make room appart from that, megga!!!!!!!!!

How you set them up is completely up to you. You can just slide the grip in some more. You might need to slide the shifters and brake levers as well if you do this. This could be uncomfortable or difficult with short bars. The other option is to trim the grips as you said but you might en up with very little grip area between the bar ends and levers. Just set them up however you find they will work the most comfortably. Don't point them up too much either. They work best when they are almost parallel to the ground. Just tilt them up a bit (roughly the same angle as your stem).