On the same Screen 11, under Adjustments of Federal Taxable Interest, as amortized bond premium.

Edited:

I overlooked that your question is about both Box 6 and Box 11.  In that case, you'd enter these amounts on Screen 11, under Tax-Exempt Interest, as municipal bonds/private activity bond interest and Amortized bond premium on tax-exempt bonds.

For AMT and state tax purposes (if that's relevant to your client), you should refer to the material that comes with the 1099-OID or obtain the relevant information from the broker, like you normally do with other types of muni/private activity bond interest.