All garments are measured laying flat on the ground in inches. Chest measurements will be from pit to pit. For the entire measurement, double the length listed. For example, a pant is listed as: Waist 13" you double it. Meaning it measures as a 26" waist. I measure the inseam from the inner part of the pants starting at the crotch to the bottom hem.
For best results when comparing these measurements to yourself I highly recommend that you compare not to your body but to a garment you already have that fits you well. Lay your garment flat and measure each level (chest, waist, and length). When a garment is laying flat vs. when it is on you gives out different results so it's usually best to try and compare it to something you know fits you that is also laying flat.