I suppose I'm an expert on this topic, since I've had to do it two years in a row. And I'm again not happy with the decision my husband made to not sell at the time of vesting, but that's another story. The stock tanked and we lost so much money, plus had to pay out of pocket for taxes, on the original amount vested. Lesson learned, and since then sell every time vested.
But to answer your question, you need to go to the brokerage website to find the actual cost basis. last year I called E-trade and they told me how to locate the info. This year I found it on my own. In 2024, we sold several stocks from different purchase dates at one time so I listed all the sales for that day using the sum of Box 1d - total proceeds. I suppose you could still do that if you sold throughout the year. The cost basis on your 1099-B Box 1e will be marked as 0. You will need to place a Check Mark for "I need to adjust my total cost basis." In my situation, we had 54 line items. I entered each individual cost basis on a spreadsheet and then entered only the TOTAL adjustment. Make sure you make it a negative. I also clicked item B noting the cost basis incorrect.
If you happened to sell ESPP, that needs to be a separate sale and usually the cost basis is covered so it will be on the 1099.
Good luck!!
