If you are electronically filing, TurboTax will print out a Form 8453 for you to use to send to the IRS. You will attach Copy B of your Form 1098-C to this form and mail it to the IRS. It has to be mailed - you can't electronically attach it.
If you are paper filing, you attach Form 1098-C in order of the Attachment Sequence No at the top right-hand corner of the form. When you paper file your return, you arrange your Forms and Schedules in that sequence order (it will likely be attached right after Form 8283)
