furumachi on "[Plugin: WooCommerce - excelling eCommerce] How to Customise checkout fields"

I'd like to change the order of first name and last name fields.

Default is First Name field + Last Name field.

But my hope is Last Name field + First Name field.
(In Japan we describe like this)

I've read the Tutorial below but could not understand it.
http://wcdocs.woothemes.com/snippets/tutorial-customising-checkout-fields-using-actions-and-filters/

I would like to get more information.

The point is;

1) What name of the file(***.php) should I customise ?

2) Where is the file ?

3) How to customise the description in detail ?

I am waiting for your answer.
Thank you,

http://wordpress.org/extend/plugins/woocommerce/