This is a tutorial on how to make a custom decal.
First, find or create an image (can be of several different formats, though the ones that I know that work are .dds and .png: I use .png files for my custom story) that you wish to use as your decal. Make sure that the image you use is a mirror image (you can just flip it in gimp or something) of what you want (think like a stamp).
Next, open the material editor. As shown below, make sure the type is "decal", the blend mode is "alpha," and the "diffuse" box is checked (don't bother touching any of the other settings). Open the file you wish to use as your decal, and then save the .mat file somewhere in your amnesia directory (for my custom story I save the decals in the root directory under textures\decals)
To place the decal, simply open the level editor, search for decals, and find the .mat file you saved earlier. select and place in your map!