Before we begin, make sure you have installed the extension.
Open up any PDF available publicly online. For example, lets use this one.
Now click on the UnderlineMe extension icon. This will open up a new page with the PDF.
Once the PDF file opens up, you can select any text and highlight as usual.
Bonus: After making a few highights, you'll see a bar with some notches on the right. This is a part of the extension and via this you can quickly move to different parts of the PDF. This can be very useful for large PDF files.
* You may be wondering why are there two steps involved. This is because extensions can't run inside a browser's PDF page. So one way of circumventing this is to open up a new page with a custom PDF viewer and load the extension in there. UnderlineMe uses this technique to enable PDF highlighting.