It depends if you are filing for one of four border states or a different state. If you are required to file Schedule OSC, then include the pages of the other state tax return that include the amount of tax due.
page 11 of 52, guidance for line 24: Credit for Tax Paid to Another State Enter the amount of credit for tax paid to another state that you claimed on Schedule OSC, Line 21. Refer to Page 29 for additional information. You must enclose Schedule OSC and a copy of each state return for which you are claiming credit. The other state’s return must show the computation of tax due.
beginning on page 38 of 52: Line by line instructions for Schedule OSC and more details about the border state computation for Kentucky, Maryland, North Carolina or West Virginia.