Hi,

Process to configure Alipay in odoo12:

1) First download/purchase Alipay from odoo appstore

    https://apps.odoo.com/apps/modules/12.0/ff_payment_alipay/

2) Install in odoo

3) Now activate debug mode, then go to invoicing app 

4) In  Account menu » Payment Acquirers » Select Alipay Payment » Click on Configure button.
5) Now in that you can configure all details like partner ID, Service etc..

6) now you can see pay option Alipay

i think you will see all configuration steps in alipay download page

Hope it will useful for you..