You will need a tape measure and possibly someone to hold one end as you measure.
Step 1: Measure the length of the RV from bumper to bumper. You do not need to include hitches in your measurements.
Step 2: Measure the height of your RV. Start at the roof and measure to the bottom of the frame. Do not measure all the way to the ground. Most covers are made to accommodate the air conditioner, so there is no need to include that in your measurement.
Step 3: Now, simply measure the width of your RV.
That's all! Be sure to write all of your measurements down as you go and use them to buy your cover. If your measurements are in between two cover sizes, get the size that is a little bigger.