For the federal amended return you have to attach any forms or schedules that changed. That would include at least Schedule D, plus Form 8949 if that form is included in your tax return. You also have to attach any other forms or schedules that changed because of the change in the amount of income. The explanation of changes is included on Form 1040-X. It is not a separate attachment.
For the state amended return, see the instructions for your state's form. If you say what state it is, someone might be able to give you more specific information. Every state has different forms and rules.
You would be subject to interest and penalties for late payment. It's your responsibility to report the correct basis from your own records, even if the 1099-B is wrong. But for such small amounts of additional tax, and only a short time since the payment deadline, the interest and penalties are not going to amount to much. The IRS and the state might not even bother to bill you for it.
