How do you use Photoshop to design a website layout?

Well i would reccomend Dreamweaver to Design websites as photoshop is more of an image editor rather than a website creator programme.

However a tip.. the best thing to do first is create a wire frame for your website...just like a simple sketch of how each page will look, decide which page leads to where...and how you want it to be.

http://www.photoshoptutorials.ws/photoshop-tutorials/layouts/how-to-create-a-professional-web-layout-in-photoshop/ 

Follow that link it will explain how to create a website layout , saves me from writing a lot :) theres screenshots which should be helpful.

Well offcourse for Mockups and Wireframes  , its quite speedy to design Mockups in photoshop and keep your design as image until client finalize.

Using Adobe Photoshop CC you can make a webdesign, then go to the layers right clik and select copy CSS. and paste it using edge code or dreamweaver