Glad you asked...
When preparing for an occasion you will need to find a theme for the occasion you are planning. Also, you will need to write the location to hold your gathering for your occasion as well. You can find more information here: http://www.bplans.com/personal_event_pla...
Then again, this answer might help you...
To write an occassion be creative in your writing. Also you want to funny because everyone enjoys a good laugh. Most importantly do not forget to tell why everyone is there.

