Cut a strip of it, put it between the two surfaces you are checking clearance between and torque to spec.
Then disassemble - the plastigauge will be smashed flat to a certain width. Using the gauge on the plastigauge wrapper, see what the clearance is between the two surfaces.
I asked for mine at the parts store (Autozone) and they had a strip of it for around a buck - make sure you know the clearance range you are wanting to measure - there are different gauges of plastigauge.
Right. just lay it perpindicular to the bearing surface. You do have to make sure you get the right range of plastigauge though. Let's say you are measuring for oil clearence and know spec is .005". You need to get plastigauge that ranges from say .003"-.009"... Good luck and let us know how it goes.