All you'd need is some new brake housing. First unclamp the brake cable at the caliper and pull the housing and cable out of the caliper. Then remove the old housing. You will need to unwrap your handlebars up to the main brake levers in order to remove both sections of brake housing. You'll then replace those pieces with a one piece section that's slightly longer (to account for the housing cut away to install the cross lever). If you are careful, you'll be able to reuse the brake cable though the job would be easier with new cables especially if the old one has frayed at all.

Removing the cross levers is just a matter of removing the clamp bolt coming up from the underside of the levers. Swing the clamp out of the way and the levers will come right off the bar.