Check out this tutorial.
There are a couple of things I do before I save for web.
I save my layout as a PSD file in photoshop elements in case I need to make any changes.
Next, I flatten my image, and do save as and save a JPEG file at fill size for printing.
Then I do the save for web which is in the tutorial.
Others may have different ways that they do these steps.
You will develop your own routine as you learn.