IRS requires details to be listed on Form 8949,
or on your own forms (e.g. consolidated 1099-B or spreadsheet) which have the same information and in the same manner as Form 8949.
Either way, if you choose to summarize, you have to mail the transaction details to the IRS within three business days of IRS accepting your tax return.
Amended return:
Select "I'll enter a summary", your return will generate form 8949 summary with the mail-in election.
since you must mail 1040X, the mail in election is implied.
Does your broker provide you a printable report of your transactions ??
If yes, attach the transaction detail after all the changed or added forms and schedules that you are amending.
If not, the IRS might accept you explanation (1040X Part III) that the detail is not available.
OR
You can get fillable PDF form 8949 from the IRS website. Use as many pages as necessary.
Exception: if you summarize Category A or Category D, Form 8949 is not needed for transactions without adjustments. No detail is necessary.
If your brokerage statement shows that your transactions are all Category A (basis reported to the IRS), you can avoid listing those three hundred transactions.
