Example:
Someone convos me and wants one of my heart dishes, but they want one in yellow and I don't have any listed in yellow. I tell them a custom colored dish will take 'however many days to complete', will cost '$' and I let them know a custom listing will be made for them to purchase and that once payment is received I will begin working on their piece. (I re word all of this to sound more professional too : )
I simply make a new listing for them that has the requested color change listed and other details about the order in the description...kind of like this custom listing I just did.
https://www.etsy.com/transaction/141830172?Now all they do is purchase their custom order and then you start working on their requests.
Does this make sense? I use to do custom orders without payment first and I always had an uncertainty feeling about each of them because I never knew if that person had forgotten or changed their mind...So now I always do payment first, work next.