I have an HTML template. What are the steps to convert it into a Drupal 6 theme?
Create a copy of a theme you want to modify - usually a blank theme like 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.
Make sure caching is off, and you can refresh to see the changes.