I'm trying to add decals to stairs. I've set up a image as plane on the $X$ axis, and all I want is blender to do is project the image along the $X$ axis. However, through all the methods I try it doesn't work. It textures the top of the steps and I end up with a triangle like texture. All I want is to have an image and then along one axis just apply it to the object
Get an angle of your object "front on" by using one of the orthographoc angles (Numpad 1 for front, Numpad 3 for side, Numpad 7 for top, or Numpad 9 to flip any angle by 180deg), and then press U (unwrap), and select "Project from view" This will make your UV map resemble a "snapshot" from that angle.
After that, connect your image texture as normal. It comes out looking like this:
If you want to stop the image from appearing on the top and sides of the stairs. you can use the respective axes from the Normal Vector as a mix factor between your image and whatever other material (I used plain white for the example). The Normal vector doesn't like being recombined without a third axes, so you have to do it in 2 stages - one for each respective axis that you need "erased":