Format :- 
                            TITLE (Bold and capital)



Message (or steps to follow) :-

Issued by :- 

It's not compulsory to follow the aforementioned format, though all these elements if present makes the poster lovely and presentable. You can keep these features in your desired places in the paper or as you wish.
I think it would be better if we design a poster rather than writing it.. :)
Anyways, here is the basic format for designing a poster :
=> the title should be catchy and creative.
=> slogans can be added to aid your content.
=> you can illustrate your poster to achieve a wider audience.
=> date, time, venue and the contact number should be mentioned in case if you are addressing an event.
=> the message should be clear and crispy
=> finally, the name of the issuer and their designation should be mentioned.