U can make it in an A 3 size sheet.
make a black colour border of thr poster about 3 cm.
on the top write swach vidyalay swach bharat.
then draw a broom below and a school then the outline of the indian map like u can make it innovative by ur own ideas. fill it with colours. write some good things about what will people get from cleanliness and write down what shall be done like plant a tree.
throw the garbage in the bin.
throw garbage in the right bin etc etc.

u can also print pics and paste them.
make it more colourful.
u can use the internet's help.

hope u liked my answer olease press the thank you button and mark me as the brainliest .:);)