How can I make a splash page for a WordPress site?
This is simple to do in a whole bunch of ways if you know how. If you’re not up on programming (especially PHP) it’s not so simple at all. Personally, I wouldn’t do this as a splash page (I probably wouldn’t do it at all, but that’s neither here nor there). I’d do it as a bit of Javascript that checks the cookie and then puts an absolutely positioned DIV over the majority of the page if the cookie isn’t found. If this isn’t something you already know how to do, though, you might be best seeing if you can find someone to do it for you for relatively cheap.
