King Juan Carlos delivered his notice to the President advising that he would abdicate. I cannot find a link for that right now, but there are photos of the two of them, and of the brief signed notice.

The Spanish will pass a law allowing or enabling the abdication, just as the British did for Edward VIII, is how I understand this.

Edit: the third pic here shows President and King shaking hands when the King handed his notice.

http://www.hellomagazine.com/royalty/2014060219106/spanish-king-juan-carlos-abdicates/