Spending several days for me to find out how to theming Input Form in CCK2 on Drupal 6, hope this can help you and save your time.

To theming CCK2 Input you just need:

1. Edit template.php
   
2. Create node-content_type-edit.tpl.php
   
3. Clear Cache Data before view your result: Administer-Site Building-Performance: Clear Cache Data

EXAMPLE

Suppose your content-type is: "account_registration" and you theme is "bluemarine"

1. Edit template.php, add this:
   
   
   <?php

function bluemarine_theme($existing, $type, $theme, $path) {
  return array(
    'account_registration_node_form' => array(
        'arguments' => array('form' => NULL),
        'template' => 'node-account_registration-edit'
    )
  );
}

?>
2. Create node-account_registration-edit.tpl.php
   //To REMOVE Title field
<?php unset($form['title']); ?>

<fieldset class=" collapsible">
    <legend>Company Data</legend>
    <?php
      //NOTE: if you don't have Field Group then simply type:
      // print drupal_render($form['field_accreg_company_name']['0']['value']);
      print drupal_render($form['group_company']['field_company']['0']['value']);
      print drupal_render($form['group_company']['field_street']['0']['value']);
    ?>
</fieldset> 
<?php print drupal_render($form);  ?>
<?php
// print_r($form);   //Enable this to show all Array Variables of Form
?>

HOW TO ...........

1. RENDER ONLY 1 FIELD
   
   
   <span style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 187);">&lt;?php </span><span style="color: rgb(0, 119, 0);">print </span><span style="color: rgb(0, 0, 187);">drupal_render</span><span style="color: rgb(0, 119, 0);">(</span><span style="color: rgb(0, 0, 187);">$form</span><span style="color: rgb(0, 119, 0);">[</span><span style="color: rgb(221, 0, 0);">'group_company'</span><span style="color: rgb(0, 119, 0);">][</span><span style="color: rgb(221, 0, 0);">'field_street'</span><span style="color: rgb(0, 119, 0);">][</span><span style="color: rgb(221, 0, 0);">'0'</span><span style="color: rgb(0, 119, 0);">][</span><span style="color: rgb(221, 0, 0);">'value'</span><span style="color: rgb(0, 119, 0);">]); </span><span style="color: rgb(0, 0, 187);">?&gt;</span></span>
2. RENDER A GROUP OF FIELDS
   
   You don't need to render fields in group one by one, just enter this code to render all fields in a group:
   
   <?php print drupal_render($form['group_company']['field_street']['0']['value']); ?>
3. RENDER A SELECT LIST
   
   Almost same like render a TEXTFIELD, but avoid ['0']['value'] at the end
   
   <span style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 187);">&lt;?php </span><span style="color: rgb(0, 119, 0);">print </span><span style="color: rgb(0, 0, 187);">drupal_render</span><span style="color: rgb(0, 119, 0);">(</span><span style="color: rgb(0, 0, 187);">$form</span><span style="color: rgb(0, 119, 0);">[</span><span style="color: rgb(221, 0, 0);">'group_company'</span><span style="color: rgb(0, 119, 0);">][</span><span style="color: rgb(221, 0, 0);">'field_region'</span><span style="color: rgb(0, 119, 0);">]); </span><span style="color: rgb(0, 0, 187);">?&gt;</span></span>
4. REMOVE AN INPUT FIELD
   
   You may want to disable an input form, usually you need to remove TITLE as shown below:
   
   <span style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 187);">&lt;?php </span><span style="color: rgb(0, 119, 0);">unset(</span><span style="color: rgb(0, 0, 187);">$form</span><span style="color: rgb(0, 119, 0);">[</span><span style="color: rgb(221, 0, 0);">'title'</span><span style="color: rgb(0, 119, 0);">]); </span><span style="color: rgb(0, 0, 187);">?&gt;</span></span>
5. HIDE AN INPUT FIELD (HIDE vs REMOVE!)
   
   You may still want to enable an input form but need to prevent it:
   
   <span style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 187);">&lt;?php $form</span><span style="color: rgb(0, 119, 0);">[</span><span style="color: rgb(221, 0, 0);">'title'</span><span style="color: rgb(0, 119, 0);">][</span><span style="color: rgb(221, 0, 0);">'#access'</span><span style="color: rgb(0, 119, 0);">] = </span><span style="color: rgb(0, 0, 187);">FALSE</span><span style="color: rgb(0, 119, 0);">; </span><span style="color: rgb(0, 0, 187);">?&gt;</span></span>
6. SHOW ALL VARIABLES OF FORM
   
   You may want to know what variables available for you:
   
   <span style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 187);">&lt;?php print_r</span><span style="color: rgb(0, 119, 0);">(</span><span style="color: rgb(0, 0, 187);">$form</span><span style="color: rgb(0, 119, 0);">); </span><span style="color: rgb(0, 0, 187);">?&gt;</span></span>
7. REORDER
   
   Use ['#weight'] to reorder
   
   $form['buttons']['#weight'] = -50; // buttons at the top
8. PRINT BUTTONS
   
   
   <span style="color: rgb(0, 0, 0);"><span style="color: rgb(0, 0, 187);">&lt;?php </span><span style="color: rgb(0, 119, 0);">print </span><span style="color: rgb(0, 0, 187);">drupal_render</span><span style="color: rgb(0, 119, 0);">(</span><span style="color: rgb(0, 0, 187);">$form</span><span style="color: rgb(0, 119, 0);">[</span><span style="color: rgb(221, 0, 0);">'buttons'</span><span style="color: rgb(0, 119, 0);">]); </span><span style="color: rgb(0, 0, 187);">?&gt;</span></span>
9. RENAMING BUTTON
   
   What is "Submit"? You may need to write it as "Save now!", don't you?
   
   $form['buttons']['submit']['#value'] = 'Save to Database';
10. HIDE GROUP FIELD-SET
    
    
    $form['group_general']['#access'] = FALSE;
11. HIDE BUTTON
    
    
    $form['buttons']['submit']['#access']= FALSE;

Ref:
http://drupal.org/node/601646