This step by step guide will help you to buy using PhonePe UPI app from Flipkart.
Add product to your card , select your address and on the Payment section Select PhonePe UPI.
Once you click on Confirm Order, Open PhonePe App and view Pending transaction.
On your app you select your, Bank and Enter MPIN and complete Payment
If you have any questions , comments feel free to add below.