if the customer pressed the button for ask "a question" about an item, and if you have the "custom orders " turned on for your shop there should be a button that says "make this a custom listing" on the right side of the conversation when you have it open. click it and you can add a picture, writeup and shipping cost. When you publish it, only your customer will see the listing and they automatically receive a convo with a link to the listing.
otherwise i select "copy this listing" make the changes to pictures, shipping etc. put the words "Custom listing for......" in the title and the first line of the description and send a link to the listing in a convo to the customer. This option is public.