Like any legal document with huge potential ramifications in your life, consulting with an attorney is probably a wise idea.

That being said, if both parties are in agreement regarding the terms they want, then they certainly would save a lot of money at the lawyer's office. I did the separation agreement with my EX and my lawyer reviewed it and just filed it with our divorce papers.

I would put together an outline of the terms or even a draft pre-nup and take it to a lawyer for their blessing off to make sure nothing was overlooked.

Not sure if it needs to be filed anywhere or not. Not every legal doc does.

You don't know if a prenup actually protects you until it is needed either by the death of one signer or for a divorce.

If it's for a divorce, you are there to testify to the validity if it is questioned in court. However, if you are deceased than your heirs are relying on its validity. Prenups drawn by lawyers are, in my opinion, less likely to get tossed.

It's always best to have worked out what you want in a prenup before you go to a lawyer, just as you would if you were writing your own and going to a notary. A lawyer though may suggest additions specific to your situation (mine did) that you may find you want included.