To add these launch commands, right click on the game’s entry in your Steam library and go to properties. Then, from the general tab, select ‘set launch options’ and paste in whichever commands you’d like to use, removing the ” marks around them.
To skip the intro videos it’s “+com_skipIntroVideo”,
to open Nightmare (though not Ultra Nightmare) it’s “+menu_advanced_AllowAllSettings 1″
and to turn off mouse smoothing it’s “+set m_smooth 0″.
There’s also a command to speed things along even more, by allowing skipping of other loading screens as soon as they’ve fulfilled their purpose: “+com_skipKeyPressOnLoadScreens 1″.