That doc will give you the correct instructions, and it's form AT&T's web site. Simply stated, forward the text spam to 7726 (SPAM). You'll get a reply back from AT&T saying they received the message content and they want you to text them the phone number where the text originated. It's all free.
I RECEIVED THIS TEXT TODAY ON A PHONE THAT I ONLY SPEAK TO MY SON ON AND HE IS IN PRISON. IT SAID I COULD GET A FREE iPAD ONLY TODAY! IT SOUNDS TOO GOOD TO BE TRUE!