In case you never did this before, this is a very simple, basic start.
1) Upload your webfonts
Upload the webfonts (which you downloaded from underware.nl) via FTP to your server. This folder should contain all 4 font formats (.eot, .ttf, .woff, .svg) for every font. Upload them in the same directory as your CSS.
2) Include the font files via CSS
The package you downloaded from underware.nl includes a _UnderwareWebfontsKit.css file. Copy-paste that complete text into your own CSS, or link the file by adding this line of code to the header of your html file.
<link rel="stylesheet" type="text/css" href="_UnderwareWebfontsKit.css">
3) Start using the font
You can now add the font to any element in your CSS, or use it directly on your website.
...<span class='MyWebFontPro-Light'>some text</span>...