For the three basic measurements (inside, outside, depth), we'll be measuring a piece of 20mm extruded aluminum framing.

Outer Measurement

The first measurement is using the 'outer' jaws. Use the flat part if possible, to avoid any skewing. Use the thumb-wheel to get a good tight grip on the material. You'll notice it's not exactly 20mm, that's from the manufacturing tolerances, not the calipers.

Inner Measurement

The inner jaws are used to make measurements of slots and holes. These are a little tougher, make sure you're holding them so you are not getting an 'angled' measurement that is larger than it should be. I usually take a few measurements and also 'wiggle' the calipers to make sure they are measuring the minimum distance.

Depth Measurement

The last basic measurement is depth, often used for drilled holes. This measurement uses the gauge at the end of the calipers. You'll want to practice how to hold the calipers to push the tapered end piece down while also keeping the tail flat against the work, its a little counter intuitive!