Question: I have an HTML template. What are the steps to convert it into a Drupal 6 theme?

Answer: 
There is no automatic way to convert your HTML to drupal theme. Easiest way to create your own drupal theme is to start with Zen theme then customizing the CSS.

Here's a link to Zen theme
<http://drupal.org/project/zen>
There's no quick easy solution. I would suggest reading the [documentation for theming](http://drupal.org/theme-guide) at Drupal.org. After getting familiar with that information, hit up the [Tools, best practices and conventions](http://drupal.org/node/341707) section specific to theming.

When it comes time to do the conversion from HTML to Drupal, I think you will find Firebug or the development tools of Chrome to be indispensable, inspect element in either tool will be very helpful.