The first thing I did was to customize the "Basic" theme. This gave me a starting point. I removed all presentational information, starting with the CSS code. Then I went through the HTML and removed presentational hints. For example, layout tables, <center> tags, characters, etc. Even things like class="left-bar" had to go. "Left" is too presentational, and I relaced it with "sidebar," since it might be argued that "sidebar" is a semantic hint. (Yes, that's how much of a nerd I am.) Fortunately there wasn't much presentation baked into the HTML to start with, because most Roller themes are pretty good about separating content and style.
Wherever possible, I made the HTML as semantic as possible; selecting and organizing
tags elements by meaning and structure. Source-order sensibilities were used, placing more important things higher in the source code. Headings/sections were arranged such that if you removed all but the headings (<h1> - <h6>) you'd get an outline of the page. <p> was used to contain all logical fragments of text. <div> was used to delimit sections. <ul> was used to build lists. It turns out, semantic HTML is really pretty brainless and straightforward, once you yank out presentational noise.
Once the blog was purified, it was boring-looking, but usable. No columns, just black Times New Roman text that strung all the way across the browser screen on a white background, with blue hyperlinks. It was nekkid! Time for some clothes. An empty CSS file was <link>ed to the blog, which I began populating, resulting in the current design.
As you may know, it's easy to do CSS design in Firefox, Opera and Safari, all of which understand and render CSS beautifully. But then you inevitably have to do battle with IE. Most designers approach the IE problem by peppering their CSS file with hacks. Hacks are oddly structured CSS commands that, due to IE bugs, either can't be understood by IE or can only be understood by IE. Instead of doing this, I used conditional comments to load corrective stylesheets into just IE. Conditional comments are an IE-only feature that look like regular HTML <!-- --> comments to real HTML parsers, but have special meaning to IE. This way, I was able to quarantine IE-specific bigfixes to a couple small, IE-specific files.