It's very straight forward though will probably involve unwrapping and rewrapping the bars:-
1. unwrap bars assuming brake cable currently runs under tape from levers
2. remove inner cable wire totally
3. position crosstop lever to suit
4. cut brake cable outer so that you have a section from the drop levers to the crosstops and then a second section from there to the calipers
5. thread cable inner back through and reconnect brakes
The crosstop works by compressing the cable outer rather than pulling on the cable inner as per the main lever. There shouldn't be any difference in performance from either lever. You may have to get new cable inners and outers if length is tight, or condition isn't great. I'd aim for a crosstop lever with an adjuster barrel on it.
If you're used to doing brake cables then it's easy, if you're not then watch some online instructional videos first or take it to the shop.