Before you start measuring... remember, size is just a number. Don't suck in or pull the tape measure tight. Stand in a relaxed position, let it all hang out and measure with a taught, but not tight tape measure. It's better to get the RIGHT size than worry about what the size is.
Measurements are of the garment lying flat.
We recommend comparing the measurements to another top/jersey or pair of shorts/pants you like the fit of.