Open the Button settings and choose the «On click» (1) option. After that, click on «Proceed to payment» (2):
Choose «Stripe», go to «Settings» and enter your «Publishable key» and «Secret key». You can find those using the guide: https://stripe.com/docs/keys.
Next, choose the payment currency and the name of the product, if needed.
Proceed to payment settings and choose the «Amount source» option:
Choose the result that will act as the source of amount to be paid by your clients.