How to add Decals? (single image)

Hi, I am having trouble adding a single picture onto my object. I fairly new to texturing and materials so If you know how to do this please explain to me step by step what I have to do to make this work. Thanks!!

Use the alpha channel of the texture as the colour mix factor. The mapping node with Min and Max enabled prevents tiling of the decal texture

You can also shrinkwrap it with the shrinkwrap modifier:

That tut doesn’t use the cycles render engine so wouldn’t be applicable for the OPs case