I legally changed my last name. Most of my tax documents have my new name, but not all of them. How do I register this on my taxes this year?

You enter the name on your tax return the same as it is shown on your current Social Security card.

As DoninGA mentions above, you file your return in the name that matches your Social Security card.  You will not encounter any issues with the fact that some of your documents have a your former name and you do not need to indicate that they are different anywhere in the program.  

If you only recently changed your name with the Social Security Administration, there is a chance that it may not yet be reflected in the systems that house the e-file data. 

Should your tax return reject with your current name, return to My Info and transmit your return with your former name to pass the e-file checks.  Everything will catch up to your new name in time, and in the meantime it will track by your social security number so that no data will be lost.