To get accurate measurements, you will need a soft measuring tape and a friend or partner to help out.
Step 1: Neck Measurement.
Measure around the base of your neck,below your Adam's apple where a shirt collar would sit. Add 1-2 fingers in-between the tape and your neck to ensure space for breathing.
Step 2: Sleeve Measurement.
Keeping your arm down at your side, start the tape at the center back of your neck, extend the tape over the top of your shoulder and down to your wrist. This number is your sleeve length.