Generally, to add a skin to your farm installation, you simply add the skin folder to your BoltWire skins directory. Then, change the name of the skin in your site.config page and refresh the page. Voila --your skin should display immediately.
Sometimes however it is desirable to store a skin for a specific field in the field, rather than in the BoltWire directory. This can make upgrades easier, for example, as you don't have to copy your custom skin from one BoltWire installation to another. BoltWire makes this easy. Suppose you have a field called mysite, and a skin called custom. Simply create a skins folder in your local field, and drop your skin into it. It should look like the following:
Next, in your index.php file, set the $skinPath variable to point ot the new location. You can also set this in a config.php file, perhaps even dynamically based on various parameters, if desired. Just add this line before calling the engine:
$skinPath = "skins";
Finally, change your site.config page as normal and set your skin to custom. It should go into effect as soon as you refresh the page.
Note: when using this method, none of the skins in the default distribution will be available to the field, unless you copy them to the local skins folder.