configure virtuemart for downloadable products

I have a joomla 1.5.17 website with VirtueMart 1.1.4 I have added several products which can be downloaded when paid for via paypal but need help configuring the process of downloading files. Each product has approx 10 different file types and I need the website configured so that the following events take place: 1/ customer finds product of choice and before adding to cart, must select which file type they need to download (choice of approx 10 different file types) 2/customer pays for chosen product via paypal and once paid, should be redirected back to website which provides link to download their chosen file. I understand that virtuemart can do this opperation but I don't know how to make it happen.