The only tool you will need is a flexible tape measure. When comparing measurements, keep in mind that fabric can vary from item to item. A precise measurement is difficult to guarantee, so you should allow about a 1/2" difference between the listed measurements and the actual garment.
Before measuring, lay the garment on a flat hard surface like a table or clean floor. Be sure you smooth the garment flat, especially the underside, to remove any wrinkles.
Bust: This measurement should be taken directly under the armpits and doubled.
Waist: For pants and skirts, measure across the very top, then double that measurement. For shirts and dresses, measure 6-8" inches below the armpit, then double that measurement.
Hips: For pants and skirts, measure 8-9" down from the waist, then double that measurement. For shirts and dresses, measure 14-16" below the armpit, then double that measurement.
Sizes vary depending on manufacturer, design and style. The size chart provides general guidelines.