"The article made mention that it is good policy to obtain a signed permission slip from retailers. I would like to learn more about how to go about doing this, and if there is a form for doing so."
Never ask permission! Ask forgiveness... That being said, as long as you avoid the 5 places you can't legally carry,
unless there's a sign on the door banning carry, I NEVER ask. If they ask you to leave, you have to leave. (don't argue with them) There are even exceptions to the 5 places you can't carry, but regarding your specific question, you do not need advance permission from a retailer.
I go into grocery stores, restaurants (that don't serve alcohol) gas stations, really almost everywhere I go. I've never asked permission and only have been asked to leave our local mall (which has an unposted policy)