It is a very  steep learning curve, and I keep forgetting what I learn, so just going to keep a list.

1. To make a link to ‘id’ on a .ctp page link to something more useful change the column name thus:

   echo $this->Html->link($powerpoint['User']['id'], array('controller' => 'users', 'action' => 'view', $powerpoint['User']['id'])); 
   
   //becomes
   
   echo $this->Html->link($powerpoint['User']['username'], array('controller' => 'users', 'action' => 'view', $powerpoint['User']['id']));

   (The query is doing a join automatically in the background)

2. To make a droplist show a useful thing like the name rather than the key, you need to go in to the XxxxxxController and change the function thus:

   $users = $this->Powerpoint->User->find('list');
   
   //becomes
   
   $users = $this->Powerpoint->User->find('list',array('fields' => array('User.id', 'User.username')));

   I guess it then used the two fields as given.  If you add a third field it is used as a label in the option.  Not sure how to concatenate the columns if you want “Firstname Lastname” in the droplist.

3. The Model reflects the database. I knew that but recording it here, as I actually used it.
4. If you have a table that is joined through another table (belongsto > belongsto) then you can make it appear thus in the Controller:

   		$users = $this->Question->Powerpoint->find('list', array(
   				"fields" => array('Powerpoint.user_id', 'Users.username'),
   				"joins" => array(
   					array(
   						"table" => "Users",
   						"type" => "LEFT",
   						"conditions" => array(
   							"Powerpoint.user_id = users.id"
   						)
   					)
   				)
   			));