To make poster of this topic, I can give you many ideas. first write the heading as beautiful as you can. Then write what are public toilets. After that draw a public toilet's outside view. Then write some slogans which you can make yourself. After that draw a man cleaning toilets and beside that man defecating in a nearby tree wearing helmet. Then you can write a catchy slogan -was Swachh Bharat really successful ? Then why ignorance to public toilets ?