How to set up PayPal Express Checkout correctly?

Question

Java

Good day to all!
I am developing a service for creating online stores (something like Shopify)
. It is necessary to implement payments. The choice fell on PayPal.
The workflow should be as follows:
1) the user creates a store with us
2) using the RequestPermission request, we construct a URL of the form:

" https://www.sandbox.paypal.com/cgi-bin/webscr?cmd=... "
+ requestPermissionsResponse.getToken()

3) The user gives us the right to execute Express Checkout, we get a token and a token_secret.
4) Next, we execute setExpressCheckout, getExpressCheckoutDetails and doExpressCheckout.
Having considered the examples from paypal's git repository , one thing is not clear to me: how, using token, token_secret, do the operations I need on behalf of the user who gave me these rights? I found such a page in the docs , where it says that you need to construct headers and then use them, but I'm not very clear how.
If you give me an example, I'll be immensely grateful :)