I made a decent amount in short-term capital gains in 2020. As I was filing my taxes this year, Turbotax informed me that I needed to pay a penalty for underpayment of estimated tax. I don't totally understand this.

Is the reason for this that I obtained capital gains during the year, but did not withhold/pay any money on them until just now? Of course my employer withheld the appropriate amount from my salary, but is this penalty saying I should also have been making additional payments on the capital gains throughout 2020?

1. How and where do I do this?
2. How do I determine the amount?
3. Isn't this a bit speculative? What if I make $25k in January of 2020, make a payment for those capital gains, and then proceed to lose $30k for the rest of the year? Is it just money I need to basically give up, and then maybe get back later if I overpaid?