Simple manual:

1) Load your texture: select "PaperDecal" group and press Tab↹ to enter
 
 2) Adjust aspect ratio with Width and Height sliders
 
 3) If you create copy of material, dont forget a "click to make a single-user copy" icon at top of PaperDecal node-group

Additional: 

• Colorize and Hue sliders mostly used to customize grayscale image
• To switch on decals Shadows:
     - for Eevee:
          Material Properties > Settings > Shadow Mode
     - for Cycles:
          Object Properties > Visibility > Ray Visibility > Shadows   
• Use "Clip Start" for control decals Overlappings:
          3D viewport > N panel > View > Clip Start
  
• See examples!