A very detailed technical specification task for the payment process - milestones on my multi-vendor site

I'd like to add milestone payments to my site The site sells digital products from different suppliers. I want to add a payment option - milestones on the site and need a precise technical specification so that I can pass on to programmers. The characterization should include - what happens in terms of the customer, when he pays and how it is a stage in the work, What happens in terms of the supplier at the time of payment, confirmation of work, etc. As well as what happens with the payment in terms of management site