I have a few questions about bringing a new gun related product to market.

Specifically, what steps must I take to be 100% legal, and, of equal importance, what order must these steps be taken in?

I have an idea for a truly harmless product. It is an accessory that is designed to enhance the overall gun-owning experience, but not necessarily the shooting experience.
I suppose it is possible that a shooter could fire their weapon with my device installed, rather than taking the time to uninstall it each time they go to the range, and then reinstall it when they return, but the item in question has nothing to do with shooting.

Having said that, I am currently in the process of trying to create a model (made of clay), as well as a prototype (more representative of the actual material I intend to use).

At what point do I need to seek ATF approval for the sale of this item?

Which part of the ATF specifically do I need to contact, and what is the name of the type of document I should seek from them? (Is it called an opinion letter? Is that the one that carries the weight of law with it?)

How do I go about finding out if my item is regulated by ITAR?

That's all I can think of at the moment, but I welcome any related info you all may have for me.
Thank you for your time.

ATF approval?  If isnt a firearm receiver or a device to operate/manipulate the trigger to induce rapid fire, why would you need 'ATF approval' of a gun accessory?

Sounds to me like you should be more focused on protecting your IP (filing for patents, trademarks, etc) so no one can steal your idea.  Get that done first.  

I've seen a lot of products that didn't meet the criteria you mentioned, yet still carry "ATF Approval".
I plan to seek the same.

The drawings for patents and trademarks will be made based on what I create on the physical model.
I have someone helping me with those concerns.
What I'm curious about is the legal stuff.

You never need to seek ATF approval, and you can read US Munitions List to see if it is ITAR controlled or not.

I'm still going to try if I can just find out what the proper name of the type of letter they write that carries the weight of law.
I need to know what that letter type is called so I can request it specifically.

Is this PDF the right place to look for info on the US Munitions List?

ETA: Nevermind, I found the section 121.1 Heading "United States Muniitons List" so I know I'm in the right place.

ETA2: Looks like my item would be considered ITAR related based on this wording "accessories and attachments for the articles in paragraphs (a) through (g) of this category."
(This item would be considered an accessory for items in categories (a) and (g).)

So now my question is how do I go about getting the product ITAR certified and how much does it cost?

I don't think you have to get the part itself certified.  If it meets the definition of a regulated item--it is.  Now, you may have to register with the State Department as a manufacturer...I would contact them.

The letter is called a 'ruling'.'

It can be revoked at any time possibly forcing you to retrieve the sold items.

Okay thanks. So in my case, I would enclose a written letter to them with my product sample and request a "ruling letter". Thanks.

Make sure you have set up some type of corporate entity to shield you from lawsuits.
It is just a nasty fact of life in our country now.
Anyone can sue anyone for anything.
Even telling someone to 'go blow' on a legal letterhead is not free.

Already done with an LLC. But thank you just the same. Other people need to read this.