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Like Paul, I design from the inside out, starting with the most important content-type on the website.
If the product page is going to do the heavy lifting in generating conversions, then that should be the main focus of my design efforts.
From there, I design a nice comfy couch for the content to sit in. When it comes time to create the homepage, most of the difficult design decisions have been made, and you can use your existing framework to build new components that will get users exploring.
This is all completely backwards from how I did things 4 years ago- starting with just a homepage mockup with dummy text, and finding out later that the most important content didn’t fit with the rigid layout implied by my homepage.