How to build PayPal digital download script

i m in a training of how to build a script for Paypal to diliver digital downloads. i went through some github tutorial but never found one that can easily use.
i see wordpress plugins like easy digital downloads can use but using paypal IPN method is best without having a plugin. and collect payment using paypal IPN while sending digital download!

do you know how?

#paypal #ipn #script