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.
If you provide me image if your theme, I could tell you some common plan for that.
Thanks for image.

my advices are
I suggest not realy zen theme implementation, because it suggest just to change css. and you already have html and css that was done not in drupal way.

1. Install any theme to your sites/all/themes. I will use for example zen theme. So path will be sites/all/themes/zen
2. Copy files from sites/all/themes/zen/zen sub-theme to sites/all/themes/zen/mytheme
3. Rename sites/all/themes/zen/mytheme/zen.info to sites/all/themes/zen/mytheme/mytheme.info
4. Change theme name in mytheme.info
5. Copy all your css and js files to sites/all/themes/zen/mytheme (better to create subdirs for css and js)
6. Remove zen default zen css files

> 
> 
> ```
> stylesheets[all][]   = html-elements.css
> stylesheets[all][]   = tabs.css
> stylesheets[all][]   = messages.css
> stylesheets[all][]   = block-editing.css
> stylesheets[all][]   = wireframes.css
> stylesheets[all][]   = zen.css
> stylesheets[print][] = print.css
> 
> ```
> 
> 

7. Add your css files to mytheme.info. Using this construction

> 
> 
> ```
> stylesheets[all][] = mycss.css
> 
> ```
> 
> 

8. Add your js files to mytheme.info. Using this construction

scripts[] = myjs.js

More info about theme.info file look here <http://drupal.org/node/171205>

1. Look at this image ![alt text](https://i.stack.imgur.com/v1jwf.jpg)

This is how I think better to split page.

Menu under header looks like primary menu. To theme them add

```
function mytheme_menu_links    ($items, $type = 'free') {
    if (!empty($items)) {
      foreach ($items as $index => $link) {
      $output = l($link['title'], $link['href'], $link['attributes'], $link['query'], $link['fragment']); /* insert your html*/
}
    return $output;
}

```

Right block looks like block. So check block.tpl.php and block theming manual <http://drupal.org/node/104319>

Content area theming depends of what we are showing as content. Usually it is view or node.
so views = <http://drupal.org/node/352970>
node = <http://drupal.org/node/11816>

All other html place into page.tpl.php. But you should do this befor theming blocks, menu or content areas.
<http://drupal.org/node/11812>