So, just like we plaster a wall by covering it completely with a flat, sticky substance, we can plaster other things onto other places. To plaster something with things is to cover all of it (or almost all of it) with things in a way that seems flat, strong, sticky, or permanent.
People have plastered flyers all over this board. That is, they've totally covered this board with lots of flyers.
If you get wet—because you got sweaty, or you got rained on, or maybe you fell out of your boat—then your wet hair may be plastered to your face, and your wet clothes may be plastered to your body. That is, they're sticking to you, wet and flat.