Here's how:
1. Go to Dashboard.
2. Beside the mutual fund's name, you will see 'Invest' written. Click on it.
3. Enter the amount and choose 'One-Time'.
4. Complete the payment. Done!
Here is a GIF to show how you can do so when you open the Groww website.
Note: For monthly SIP, it is best to have an approved AutoPay mandate for automatic monthly payments.