A Tape Measure (with millimeters).
Like the ones used by tailors. It has to have the millimeters.

Two assistants.
One for placing the tape measure and one to take the picture.

A camera with good definition (8 megapixel at least, use the best camera you have).

Camera, Reflex, Smartphone or Tablet (iPhone 4 o better, Galaxy S5 or better, check the characteristics of your device on the website of the producer.)

WARNING:
Low definition pictures will be rejected.

Underpants (or swimwear).

Do not use boxer.
Do not send nudity.