Here's how you can do that:
On the website: https://youtu.be/z0NJcABDkFU
On the app:
Home page → Mutual Funds (either from the center of the screen or from the bottom tab) → The goal in which you want to invest lumpsum → Invest → Invest Lumpsum → Fill up the amount, risk level, etc. → Review and Confirm Lumpsum.
Note: If you have not invested in Stocks & ETFs, then you will be on the Mutual Funds page by default.
If you wish to invest in the funds of your choice, refer to this FAQ
If you wish to check by when your investment will get processed or reflect on your dashboard, you can refer to this FAQ.