Use a tape measure that is numbered only on one side to avoid possible measuring errors (blot out one side with a felt pen if necessary). Tapes can stretch, check periodically against a yardstick.
When taking measurements for a custom wetsuit customer should be in a bathing suit. When taking measurements for a custom drysuit, customer should be in undergarments to be worn under the drysuit when diving. For circumference measurements tape should be snug but not pulled tight. Make crotch measurements with a pen or ruler held at the customers highest crotch point. Recording the customers height, weight and age is very important. A brief comment on the customers physical condition would be helpful.