There are two prepositional phrases here: over the hill is the first one, and the second one is through the garden. Prepositional phrases, as the name itself says, are phrases which start with a preposition. So in order to find prepositional phrases in a sentence, you first need to locate the prepositions. In the sentence above, the prepositions are over and through, which means that the words which follow them make a phrase with them.
