To make: made just like a standard texture, except that the TGA needs an alpha channel. You'll need Photoshop, PSP, or some other good image editor for that. The VMT will need "$decal" 1 and "$decalscale" 0.1 or so ($decalscale controls the default scale of the decal, for reason of letting high-quality, large images be small decals).
To use: place infodecal or info_overlay entities, or use the decal/overlay application tools. They're on the big toolbar.