Here's how you can do that:
On the website: https://youtu.be/H1qo09F97LM
On the app:
Home page → Mutual Funds (either from the center of the screen or from the bottom tab) → Click on the goal in which you want to set up the SIP → Invest → Start SIP → Fill up the amount, duration, risk level, etc. → Review and Confirm SIP.
Note: If you have not invested in Stocks & ETFs, then you will be on the Mutual Funds page by default.
Please note that you will be able to set up a SIP from your dashboard up to 1 day prior to the scheduled SIP start date. For example, if the scheduled SIP start date is January 7, you can set up the SIP till midnight on January 6.
If you wish to invest in the funds of your choice, refer to this FAQ