Need to integrate payment gateway via web services

[rvkvino]

I have created web services in Prestashop 1.7 upto order placement. Now I have to integrate payment gateway to pay the payment for customer orders. I have installed the payment gateways Payu and paynimo Techprocess payment gateways in prestashop admin. This is working fine in website. Now I'm creating the  mobile app for this creating the webservices for all the process of mobile screen. I need some help on doing payment in webservices. I need to know the steps of activity for this achieve successfully.

I means whether need to post order creation whether after successful payment or first I need to post order creation and then track to payment screen by this if payment get successful to update in order status. Please anyone help me on this best way of doing this one.