Tighten the screw with the clothespin until the clothespin slips and the end of the screw is up against the fender washer. The zero mark on the dial should be lined up with the reference indicator. Loosen the screw a little less than one whole turn (until the zero on the dial is almost to the reference mark again) and insert a piece of paper between the end of the screw and the washer. Tighten the screw again, once more using the clothespin until it slips and the end of the screw is up against the paper. Then read the dial.
Each mark on the dial represents about one one-hundredth of a millimeter, so the mark that lines up with the reference indicator indicates the thickness of the paper in hundredths of a millimeter. Ordinary copy paper usually has a thickness of between ten to fifteen hundredths of a millimeter (10 to 15 on the dial).
Try measuring other objects. Some things to consider are an index card, a paper clip, a piece of thin spaghetti, regular or heavy duty aluminum foil, nylon fishing line, wires of different gauges, plastic bag material, or even a human hair.
Periodically check to make sure the reference indicator still reads zero when there is no object in place. If it doesn't, then recalibrate the dial.
If the object you’re measuring is larger than one millimeter, you’ll have to keep track of how many turns you have to loosen the screw (and therefore the dial) before measuring the object, and then add the appropriate amount to the dial reading. Each complete turn of the screw moves the end of the screw one millimeter.
You can increase the reliability of a measurement by doing multiple trials and finding their average, rather than relying on a single trial.