Sorry but there is no step by step instructions to make a skin from a design.
It requires knowledge of html and css and a strategy based on that knowledge proceeding step by step its a trial and error process. Each template is different so the strategy varies.
The templates have css selectors that are different than what is rendered in mojoPortal, so its a matter of harvesting the style rules and adding them to or creating appropriate selectors that work in mojoPortal.
Best,
Joe