Making blocks multiblock enabled

The readme file included with the module explains how to do it. But here's how and some very basic example code:

Implement hook_block.

Add an if statement to handle the op 'mb_enabled':

if ($op == 'mb_enabled') {
  return 'mb_enabled';
}
else if ($op == 'list') {
  // etc...
}

When $op is 'view', 'configure', or 'save' use the multibox delta in $edit to effect the right block instance.

if ($op == 'view') {
  $instance = $edit['multiblock_delta']['#value'];
  // $delta is the block type number that the normal hook_block uses.
  $data = variable_get('mymodule_block'.$delta.'_'.$instance, null);
  // change $data however you want here
  $block = array('subject' => 'Subject', 'content' => $data)
}

Now that your hook_block is multiblock ready you can start cloning it on the admin/build/block/instances page.

When I tried multiblock myself it wasn't saving different settings for each block. I found some bug fixing patches for multiblock that aren't part of the download on the main page yet. Replacing multiblock_add_form_submit (in multiblock.module) with this seems to fix the problem if you ever have it:

function multiblock_add_form_submit($form, &$form_state) {
  // Get the original block info.
  $orig_block = multiblock_blockinfo_from_form($form_state['values']['block']);
  // Create block instance information.
  $orig_block = (object) $orig_block;
  // Check whether this module is multiblock enabled.
  $mb_enabled = (int) (module_invoke($orig_block->module, 'block', 'mb_enabled') == 'mb_enabled');
  $instance = (object) array('title' => $form_state['values']['title'], 'mb_enabled' => $mb_enabled);
  // Add the block instance.
  multiblock_add($orig_block, $instance);
  drupal_set_message(t('Block instance %instance created.', array('%instance' => $form_state['values']['title'])));
}

Thanks for taking the time to explain this ocarina, I really appreciate it.

where do you add the if statement?

Where do we have to "implement hook_block"? In the template.php?

In a custom module