~$ cd /var/www/your-site-folder/wp-content/
~$ svn export http://i18n.svn.wordpress.org/tools/trunk/ makepot
~$ cd plugins/your-plugin-folder/languages
~$ php /var/www/your-site-folder/wp-content/makepot/makepot.php wp-plugin ../
With the last option you need to specify if you are creating a POT for a plugin or for a theme (for themes, use “wp-theme” instead of “wp-plugin”) and the last command line parameter is the plugin/theme folder (in this case ../ is pointing to /var/www/your-site-folder/wp-content/plugins/your-plugin-folder).
The resulting POT file will be saved under the “languages” folder (which of course must been already created).