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

Answer: 
Create a copy of a theme you want to modify - usually a blank theme like [zen](http://drupal.org/project/zen) works well. You'll need to rename the files and fix the .info file.

Then you can edit the `.tpl.php` files. `node.tpl.php` is the main skeleton one. Start copying content from your html template into that file, replacing dummy content with placeholders (which you can find [here](http://drupal.org/node/11812).

Make sure caching is off, and you can refresh to see the changes.
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.