To buy Bitcoin instantly on CoinDCX, go to prices and click on Bitcoin. It is here that you can directly press the buy button or click on the View Price Chart option to see how Bitcoin has been performing over the last couple of days/months.
Click on the Buy button and based on the current market price, select the amount of INR that you would like to spend to purchase Bitcoin of equivalent amount. Once that is done, click on Buy BTC and your order will get executed instantly.
You can now view your Bitcoins in your Investments and all your latest orders in My Orders.