Set up the rotameter on a flat surface. It is important for the meter to be level. You may need to use clamps to attach the rotameter to something sturdy; some are too large to stand on their own.
Connect the gas source through tubing to the rotameter and turn on the gas. Some rotameters come with standard fittings for various size tubing. You may need to purchase tubing connections from a hardware store.
Find the ball floating in the middle of the rotameter column. Look at the bottom of the ball, and see where it lines up with the scale on the side of the meter. This is the flow rate. Rotameters are marked from the top to bottom, increasing to decreasing flow, similar to a thermometer. The scale will vary based on the make of the rotameter.