Slice the PSD: PSD file in small pieces with several layers, each section can be flexible html block.
HTML: Write the HTML which is a markup language for web pages, initially all the section or block will be a static HTML and later while converting to DNN it can be a Dynamic Content coming from DNN modules.
Style files (Cascading Style Sheets): Along with HTML scripts a set of style files need to create that is responsible for color and style of various HTML elements.
Responsive: Web pages need to support different devices like mobile, tablet, desktop etc. Finally, pages need to be rendering well on a variety of devices and window or screen sizes from minimum to maximum display size with help of Media query or supported framework to make pages with responsive content. After Approval on HTML, we smart HTML to DNN theme phase.