Look at the sentence and decide what it is about. If you are having trouble with this, ask yourself who or what the action happening to. For instance, if the sentence is "Amy read the book to the cat." the subject is Amy. Why? Because she is the one doing the reading.
Find the verb connecting the subject to what was done. In the example above, we know that Amy "read" which is the action of the sentence.
Determine who or what the recipient of the action is in the sentence. Using the same, you may ask yourself "who or what did Amy read?" The answer is a book. Which is your direct object.
Find who or what in the sentence the action was taking place to or for. Though the direct object is Amy's book, the book was read to a cat. Which means the cat is the indirect object and is the recipient of the action.