Well, to answer your Q, there are all sorts of ways but the simplest is to have the item you want to adjust on its own layer, then just mess around with the Opacity on the Layers pallete to adjust it, or, when your highlighted tool is the Move tool, just hit any number 1-0 on your keyboard for 10% incremental shifts in opacity.
A quick and cheesy way to make thing appeared "stamped" is to go to Layers > Layer Style and apply a layer style (in this case a style of emboss) to the layer.