So if I want to use Photoshop I would have to do a file 24″x72″ at 300dpi?
Correct, but you only need 150 DPI. 🙂 Just create a new file with those dimensions, add your text/vector graphics (making sure to leave at least an inch of margin all the way around to keep any important info from getting cut off) and then export to whatever file format your publisher desires: PDF, TIFF, etc…