This double underline code is actually a part of a CSS code, so if you are using CSS stylesheet, just paste the code inside the style tags somewhere.
Step 1: Start by copying the code under and paste it on the page you want the code to work on, or as I said; inside your CSS code.
Step 2: Where it says color: #000000 you can change the color of the underlined text. Where it says font-family: tahoma; you can change the font of the text and where it says font-size: 7pt; you can change the size of the text. The double underline part is the last part of the code, the part saying border-bottom: 1px solid #ffffff;, Where it says Solid you can change it to maybe Dashed, dotted or double and where it says c0c0c0 you can change the color of the second underline. You can also change the thickness of the second underline where it says 1px.
Step 3: To make double underline in your text just use the simple code under on the words you want to underline and the underlining will become double. I have used the double underline code on this page as you probably can see:)
If this helped, a link back is required.