1) Come up with a slogan idea! I have a few examples shown and you can go to google and find many more.
2) Get your supplies. You might need:
Snacks (because you can't make a poster without having a snack :D )
3) Plan out what your posters will look like, draw them out and plan them greatly! This step insures that you know what your doing and that you won't mess up.
4) Get started on your posters, draw them out first and then go over it with paint or markers.
5) Don't get upset if you make up and don't go nuts over making them perfect.
Hope this helped guys! Thanks for viewing this and I look foward to seeing your comments!