Assume we're measuring a horizontal pipe which we're looking along. Place the micrometer in the pipe vertically with the "number" end up. Extend the micrometer until it's nearly as wide as the hole. Keep the bottom of the micrometer in one place. You can tell when the micrometer is not extended enough if you can rock it from right to left in the pipe. You can tell you haven't extended it TOO far because you can still rock it forwards and backwards ... although there should be some slight resistance when you have it just right. So you extend it slowly until it can't be rocked left and right, all the while checking that you can still rock it forwards and backwards. When you have it so that it won't go left and right, but will still (just) go forward and back you have the measurement. Lock the reading, if your micrometer can do this (some have a thumb screw), and remove it from the pipe. Reading the value: There are two tricks that some micrometers use for increasing accuracy. First they have a main scale along the tube, which you read the same as a ruler. Trick 1: the rotating sleeve sometimes has a reading on that as well, the sleeve has an expanded version of the main scale and you should be able to see that the larger divisions on the sleeve match with the smaller divisions on the main scale but are like a magnified view. If the main scale is marked so that multiples of ten are long lines, multiples of five are shorter lines and units are shorter still, then the sleeve will probably have the same pattern, only larger, and the digits shown may be the next digit of the measurement. Trick 2: Some micrometers have a cunning idea where a small scale with a slightly different pitch is placed along a scale. It usually has exactly eleven lines. As you turn the micrometer slowly, one of the lines will on the small scale match up with a line the larger scale, then another, then another. The small line indicates the next digit of the measurement. The line it matches to is not important. You have to try turning the micrometer to understand the relationship between the scales on your particular micrometer: watch what happens on the other scales when you turn it from one unit to the second unit on the main scale and note how big the divisions on the other scales must be compared to the main scale. For example, if you turn it from one unit to the second unit and the sleeve scale claims you've been through 100 things, then those 100 things are hundredths of a unit. EDIT: I thought of some more things: take several readings and throw away the wild ones to get a better answer. Wear thick gloves! This is to stop the heat of your hands throwing out the measurement by expanding the micrometer!

you could try goggling it and it will show you how to do it