My comments are based on the version released for 1.3.x

The contribution contains a .txt file which explains the installation steps in detail. I've summarized them below.

1. a stylesheet_new.css was supplied. You only need the bottom section of that file denoted as required for order-steps mod. Look in the code comments to see which section to copy from there into your own real stylesheet.
Your stylesheet may or may not be named stylesheet_new.css.

2. in the images folder, you need to choose an arrow. There are several to choose from in the optional_arrows folder. Pick one and name it "arrow_checkout.gif" and upload it to your custom template "images" folder

3. each of the 4 template files has some minor changes ... just an added section of code ... to be merged into your own template files.