This is how I created the white incised lines in my Sing & Sting pieces which was not my favorite to work with (but of course it's the one I used for all of these videos! >:( )
I'm using a sharp tool to cut lines into the wax. Then I'm using a paintbrush and white wax to fill them in. Last, I'm using a sculpture tool to scrape away the excess wax to reveal a crisp line.