I just did this on my surface laptop 3. There is a bit of surgery that you have to do. The old keyboard has a backlit sticker sheet on its back that you need to peel off carefully so that you can reuse it. And then you have to take off the metal shielding on the back of the mouse pad. The biggest pain is that there are about a hundred little rivets holding the old keyboard in place. You can pry it off and all those little rivets break off. It will completely destroy the old keyboard, but given that your replacing it it's not a problem.
Then you can put on the new keyboard and then re sticker the lighting sheet back onto the back of the new keyboard. Given that the rivets are gone, I used tape around the edges to hold the keyboard in place until the back was done. It is all working right now. Make sure to reconnect the two little ribbon connectors for the lighting sheet and the keyboard carefully.