Child Theme for the SP MDL WordPress theme.
Simply download the zip and upload the zip (sp-mdl-child-theme.zip) under your WordPress dashboard at Appearance > Themes. Or extract and upload via FTP at wp-content/themes/.
You can of course rename the zip file so it isn't called sp-mdl-child-theme.zip (you should do this so it makes more sense) and also change the "Theme Name" at the top of the style.css file.
When using a child theme (see http://codex.wordpress.org/Theme_Development and http://codex.wordpress.org/Child_Themes), you can override certain functions (those wrapped in a function_exists() call) by defining them first in your child theme's functions.php file. The child theme's functions.php file is included before the parent theme's file, so the child theme functions would be used.