2017 taxes filed. Just received a 1098 in mail today that I had totally forgot about. How do I claim that interest?

Since you have transmitted your returns, you will need to wait for them to be accepted or rejected.

If rejected, you may correct and resubmit.

If accepted, you'll have to amend (available late January or early February 2018).

Amendment instructions can be found here when the form is available*: https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

Note:

• Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.
• You're free to cash your refund check or spend it once you have it. You don't need to wait for your amendment to finish processing, which can take another 3-4 months.
• If you owe money after amending, you'll just include payment with your mailed amendment form.
• And if your amendment results in a second refund, you'll get a check for the additional amount.
• An amended return, Form 1040X, can only be printed and mailed to the IRS, it cannot be e-filed.  
• The IRS will take 12 weeks or longer to process an amended tax return.
• 3 weeks after you mailed your amendment, you can start tracking its progress at the IRS Where's My Amended Return? site.

If you have already e-filed or mailed the return, then you will need to wait until the IRS has accepted or rejected that return before you can make any changes.  

• If they reject it, then just make the needed changes (add 1098) and re-submit the return.  
• If they accept it, then you will need to wait until you have received your refund and then amend that return to make changes (add 1098). You will need to send in any forms that caused the need for an amendment (Original 1040 Form, new 1040X Form and your 1098 form). Print, sign and mail your forms to the IRS.

Click here for instructions for amending a return.