Before you begin
Use a soft, nonstretch measuring tape. Stand normally in a thin shirt or undershirt, keep the tape level, and write down the number you actually see. Do not inhale like you are posing for a driver's license photo. If you cannot see the tape behind your back, recruit a second person. Your mom has been waiting for this call.
Measure twice, record in inches to the nearest half inch, and label every number. A 17-inch neck is plausible. A 17-inch chest suggests the tape was around a traffic cone.
Neck
Wrap the tape around the base of your neck where the collar would sit, just below the Adam's apple. Keep it snug enough to stay put and loose enough to slide a finger beneath it. Record the body measurement, not the size printed on a shirt that has been through twelve dryers.
Chest
With arms relaxed, run the tape under the armpits and around the fullest part of the chest and shoulder blades. Keep it horizontal across your back. Let someone else read it. The mirror is an unreliable witness and also frequently too flattering.
Midsection and sleeve
Measure around the fullest part of your midsection where the shirt must close, often near the belly button. Do not substitute your trouser waist or your aspirational waist from 2018. For sleeve length on this form, start at the shoulder point, follow the outside of a slightly bent arm, and end at the wrist bone where you want the cuff. Different shirtmakers use different sleeve conventions, so confirm their diagram before placing any real order.
The sanity check
Repeat each measurement. If the readings differ by more than half an inch, resolve the difference before pretending the average is science. A comfortable finished shirt also needs ease beyond body measurements; do not add your own mystery allowance unless a maker requests it. Our form is a parody and sends no measurements or orders.
Return to the shirt form. Bring your measurements and an amount of self-awareness your tape can verify.