When you purchase the payment plugin, you will receive a zip file called os_dms_stripecheckout_unzip_first.zip. Unzip that file, you will get the file os_dms_stripecheckout.zip. Go to Documents Seller -> Payment Plugins to install it as any other payment plugin. See https://dmsdoc.joomservices.com/basic-setup/payment-plugins#install-new-payment-plugin for installation instructions if needed.
https://domain.com/index.php?option=com_dms&task=payment_confirm&payment_method=os_stripecheckout
(Of course, need to replace https://domain.com/ with your website url)
Stripe will also asks you for the events for webhook, select event below:
checkout.session.completed