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 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>