You can call your parent collection model like
$collection = Mage::getModel('fieldsmanager/orders')->getCollection();
Note: Make sure your namespace/entity is correct.
Then from the main collection you can join the
Here in first parameter you pass the table you want to join with.You can use an array of the format
array('alias' => 'namespace/entity')
In second parameter you add your condition to join with.
and in third one you pass the columns you want from join.
"*" resembles for all columns. If you want specific column you can pass in that array.