This isn't my typical way of working these days. User experience & flow, UI & wireframing, design, mark up, styling and interactivity are all very iterative to me. They don't come in a waterfall sequential order anymore. Working in a lab team, I concentrate on seeing whether an idea, product or technology to solve a problem is useful, before it goes into production, so I usually prioritise content, UX flow and interactivity, before a finished perfectly styled site or app.
Don't get me wrong, I do think styling is important, I just have to weigh up priorities with time constraints. I also built some starter files (more about these later) to help me along the way with styling, so it doesn't fall by the wayside entirely.
Sorry I'm getting side tracked...
...back to the design handed to me. It's a lovely design, it will probably go into production and be released. I'm ok (in this instance) with being given it and asked to turn it into a web page. It has reminded me of my workflow in this situation and something I advise people when they ask 'where so you start with a design like that'. It's a rich design. There's a big interactive hero piece.
Where do you start?
Start with the content. Start with the bones, the HTML. To me it's the crux of the whole piece. You want to just write what you see. Keep it clean and simple, use the correct elements for the correct content. Consider, but don't get hung up on, the CSS and JavaScript to come, if you need to edit it slightly later - edit it slightly later, no biggy.
But remember, there are people out there that will just see or hear this HTML and nothing else. So make it happy HTML, make it clean, lean and healthy. Don't make it heavy and bloated, all the other languages that join the party later won't thank you for it.
Disclaimer: Although this is easy for me to say when all I have to do is hand code one web page, without grid systems, frameworks or libraries. I do understand bigger projects benefit from these things, but I also believe you can't go far wrong if you keep your HTML happy.