Measure the lamp from the bottom of the base to the base of the socket. Another way to visualize this is to measure the height of the portion of the lamp you want to remain visible when the shade is on the lamp.
Select a lampshade with a bottom diameter no more than more than two inches larger or two inches smaller than the height of the lamp base. For example, if the measurement in the previous step was 12 inches, the bottom diameter of your lampshade should be between 10 and 14 inches.
Measure the height of the harp, the heavy wire arc that holds up the lamp shade. Select a lampshade with a height no more than more than 1/2-inch larger or 1/2-inch smaller than the height of the harp. For example, if the harp measurement is 9 inches, the height of your lampshade should be between 8-1/2 and 9-1/2 inches.
If your lamp has multiple bulbs, or "clusters," measure from the bottom of the lowest socket to the top of the clusters and your height should be no more than 1/2-inch larger or 1/2-inch smaller.
If your lamp has one bulb and does not have a harp, measure from the bottom of the socket to the top of the bulb and add three inches. For this type of lamp, you will also need to measure the diameter of the top of the socket. You will need a lamp shade with a ring this diameter to slip over the socket. Lamps of this type may also be equipped with hardware that will allow you to add a harp.