How to modify the search index? There's a search index for the Apache Solr module. However, I don't see any way to change this index. It seems that various fields and items are added to the index without rhyme or reason.

If I wanted to index a field called "keywords" how would I go about this? Do I have to use the method described in Adding Customized Fields to the Apache Solr index in Drupal?

Basically, there's a keyword field, which will have comma separated values of keywords, and I want to index this for Apache Solr search.
You should use hook_apachesolr_index_document_build in your custom module. This hook helps to extend basic SOLR Document object and adds custom fields for indexing.

If 'Keywords' field has machine name 'field_keywords' hook implementation may be as below

/**
 * Implements hook_apachesolr_index_document_build().
 */
function MY_MODULE_apachesolr_index_document_build(ApacheSolrDocument $document, $entity, $entity_type, $env_id) {
  if ($entity_type == 'node') {
    // Adds 'Keywords' field as a separate field to Solr index.
    $document-&gt;keywords = isset($entity-&gt;field_keywords[LANGUAGE_NONE][0]['safe_value']) ?
      $entity-&gt;field_keywords[LANGUAGE_NONE][0]['safe_value'] : '';
  }
}