Once you have created the order, you can use the order to initiate payment. You can either use KwikPaisa NEO Bank - Pg user interface to capture payment details. You will do this by redirecting the customer to a KwikPaisa NEO Bank - Pg hosted page, which can be customized for your needs.
The merchants can also build their own interface to capture payment instrument details (card numbers, UPI id etc) and call this API to initiate the payment. This API can be safely called from any device and does not require any authentication.
PG Hosted Checkout Page
If you want to use a pre-built checkout page, then you just need to use the payment_link parameter value from the create order response and redirect the customer to that page.
create order response
"payment_link": "{api_base_url}/payment/BtJEHHxOB9bFpNsaHmEL"