A promise ring is mostly symbolic, a token of the promise that you are making. The actual ring itself does not need to be specifically designed for the purpose of making the promise, although these are certainly available.
To try and make things clearer I will refer to examples of Promise Rings in popular culture. In an episode of The Simpsons (Season 3, I Married Marge), Marge’s wedding ring is re-possessed and Homer eventually replaces it with an Onion Ring from the Gulp ‘N Blow where he is working to make money to support Marge, who is pregnant with Bart. (He promptly removes the onion ring and eats it as it is burning her finger).
In this example, Homer used the onion ring as a symbolic representation of a real ring to reinforce the promise he had made to Marge. It was a promise ring.
In The O.C., Kirsten hides her wedding ring for some stupid reason and tells Sandy that she thinks it is stuck down the sink. Anyway, it reminds him of the promise ring that he originally gave her, which he won from a machine.In these instances, the sentimental value of the ring outweighed the fiscal value.
In Buffy The Vampire Slayer, Angel gives Buffy a promise ring. When you wear the ring with the heart pointing towards you it symbolises that you belong to somebody. In this instance we have a ring that in itself is symbolic of the promise and does not cost thousands of dollars.
So now that we’ve seen that almost anything can be a promise ring, how do you pick the perfect promise ring? You can look for a ring that in itself has a meaning, either culturally, or a meaning that the two of you will understand and appreciate. When you propose with a promise ring, the meaning behind the ring is what will make it valuable. As for how you do it, using the promise ring in lieu of an actual ring seems to be the way to go. So, the classic one knee will most likely suffice.
I would argue that in an engagement the fiscal value of the ring isn’t as important as the love that the two share. Use of a promise ring should be the same as a full engagement ring. I would however steer clear of foodstuffs and perishables. After all, the engagement is about making a promise, that the person you are proposing to is the person you wish to spend the rest of your life with.