Jason had a birthday party at his favorite Mexican resturant.

http://teachro.publiccomputingservices.org/diagramming/diagramming.htm

Jason | had | party

The two adjectives modifying party are a and birthday.

The prepositional phrase modifies had.

That sounds like a fun occasion! I'm happy to hear that Jason enjoyed his birthday party at his favorite Mexican restaurant. Is there anything specific you would like to know or discuss about the event?