I filed my federal in February, and then I ended up getting a 1099-MISC in March. Even though they sent the 1099 ridiculously late, I'm assuming I'll have to file an amended?

The only thing I need to amend is to add the 1099-MISC which includes 1 thing on Line 3 (Other Income).

What are all of the lines that I'll need to adjust? Thanks so much.

For 1099-Misc income you will file schedule C (Form 1040). You can deduct business related expenses. Your income on schedule C is subject to SE tax at 15.3%.

Read: Your U.S. Tax Return: W2 or 1099: Employee or Independent Contractor.