Measurements should preferably be taken by another person. Always stand up straight and look straight ahead. BE NATURAL - Relax, and do not inhale and hold your breath or exhale and hold.

CHEST - Measure around the fullest part of the chest (this is generally across the nipple), be sure to keep the tape up under your arms and around shoulder blades. If you will be wearing a thick jumper underneath your jacket, then take the measurements wearing that jumper.
WAIST - Measure the waist keeping one finger between tape and body (The widest point is usually across the navel).
BREEKS INSIDE LEG - If possible, take a pair of breeks that fit you well and measure from the crotch seam to the bottom hem of the leg (Breeks finish just below the knee).

Here is a complete set of sizing charts (not all products are available in all sizes - refer to individual product pages) ...