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How do I transform a LESS file into CSS?

Consider this bit of code to automatically enqueue your stylesheet from your theme (or plugin):

wp_enqueue_style('mytheme', get_bloginfo('template_directory').'/style.css', array(), '', 'screen, projection');

To make it process by WP-LESS, switch to this way:

wp_enqueue_style('mytheme', get_bloginfo('template_directory').'/style.less', array(), '', 'screen, projection');

You understood well: you just need to change the extension of the file.

And if I don't use the wp_enqueue_style method?

For the moment, it's the unique way to handle this.
Helpers will be provided soon to include LESS files in your templates in a fluent way.

What if a *.less file contains only pure CSS?

Nothing special. The LESS parser is fully compliant with CSS syntax.
It means nothing will be broken so don't worry.

I'm a themer and I don't want to ask my users to activate this plugin

It's a very good moto. Since the 1.1 release, there is a special bootstrap file: bootstrap-for-theme.php.
Everything is prepared and documented inside, with examples and hint.

Just help yourself!

I want to inject custom variables

LESS PHP 0.3.0 introduced a native way to set variables from PHP.

If you initialized the class by yourself, do it this way:

$WPLessPlugin->addVariable('@default_color', '#fff');

If you don't manage the plugin by yourself:

WPPluginToolkitPlugin::getInstance('WPLess')->addVariable($name, $value);

And if you want to do that from a theme, with less code:

1. include the wp-less/lib/helper/ThemeHelper.php file;
2. call less_add_variable('@default_color', '#fff')

I want to create a new custom LESS function

LESS PHP 0.3.1. introducted a new way to register functions without subclassing the compiler.

If you initialized the class by yourself, do it this way:

$WPLessPlugin->registerFunction('double', 'lessphp_double');

If you don't manage the plugin by yourself:

WPPluginToolkitPlugin::getInstance('WPLess')->registerFunction('double', 'lessphp_double');

And if you want to do that from a theme, with less code:

1. include the wp-less/lib/helper/ThemeHelper.php file;
2. call less_register_function('double', 'lessphp_double')

Notice: in this example, we assume lessphp_double is a valid PHP callback, as stated in lessphp documentation.