tax penalty fir for under payment

In the past 3 years,  we (married filing jointly), have had refunds from our tax returns.  Because of this, we did not pay estimated quarterly tax payments for 2025.  

 

Now, as of September 21,   it appears that we will have capital gains for 2025 that we did not anticipate. 

 

How do we avoid a penalty for underpayment of taxes for the 2025 filing?