You can do it either way. If you have a lot of transactions, you will probably want to report a summary of your sales. You would report them under the Sales section Long-term basis not reported to IRS. Enter the total in the proceeds and the reported cost (if any). Then click the I need to adjust my total cost basis as instructed above, using code B. You will want to adjust it with a negative number. If you choose to do it this way, you can attach a pdf of the 1099-B to your tax return once you are done entering the sales.
You may also enter the transactions individually. Select Stocks as the investment type and choose to type it yourself. After answering a few questions about your sales, select One by one in Now, choose how to enter your sales. Enter each as Long-term basis not reported to IRS or Unknown term basis not reported to IRS, as reported on Form 1099-B. You will put in your acquisition date you have on record, as well as the proceeds and cost as reported on 1099-B then select The cost basis is incorrect or missing on my 1099-B. After one more screen, you may enter your actual cost basis. If you adjust each sale individually in this manner, you will not need to attach a copy of your 1099-B to your return.
