Hey Scarlet, here's a simple method for those who don't have overhead projectors. It'll cost a little more in printer ink and paper, but it's super fast once you get going. The easiest way is to use Word or Powerpoint and put a single large letter on each page. Optionally, you can add graphics.
Simply print (in draft mode to save ink) and paste on cardboard with glue stick. Bonus tip: Use high-visibility, neon paper for that extra wow-factor.
A more advanced technique uses Publisher software. This allows you to have words/letters span page breaks, but unless your printer prints to the edge of the paper, you will need to trim the overlap paper when aligning the text. In Publisher, choose a "Banner" publication and enter the dimensions that most closely fit your sign dimensions.
Enter your text and any graphics so that they fit your chosen size. I prefer to use "Word Art" for my text because it resizes really well (Insert Picture Word Art). Be sure to do Print Preview before printing!
After printing, you'll need to trim the overlap space, and paste with a glue stick. This is a little more work, but the results are great.
Peace, Love and Free Speech,
Maria in AZ