In up and down mirror writing, some letters are the same in the mirror. If you write them, some letters are not. Instead, you have to draw them upside down to make the mirror writing. Letter that have vertical symmetry in mirror writing are C, D, E, H, I, K, O, and X.
In the picture, I put check marks by the letters that have vertical symmetry. I put the pencil behind my ear so I could look like a writer.