Here I have dry clamped the back plate to the sides of the uke and I'm using t .05mm pencil to mark the location and width of the braces so that I can use my dremal to rout the channels that they will fit into.

This is important because if the braces aren't bonded with the kerfing they essentially are just floating in space doing nothing. there are exceptions to this but X braces in X braced guitars at the very least have to have a structurally strong connection to the sides.

another funny side note is that this is not a common practice on most instruments you see. Most guitars have a binding strip that covers up braces that were actually jutting through the side of the instrument when it was glued! It's way easier to just use that method but since I lack the funds for a rabbiting bit at the moment I'm not putting binding on this one. Therefore putting the braces through the sides would look hideous.