Watch the video below. It will explain step-by-step how to view headers in Yahoo mail.
Out of all the email clients Yahoo mail is probably the easiest to for viewing headers.
Step 1: Open up your Yahoo mail window. Right click on any message. Select "View Full Header"
You will then be presented with the headers window that looks something like the image below
The email header analysis section allows you to further examine the headers. You can find out the approximate location of the user by running a quick check of the Yahoo mail headers.