This is a very well written articel, I strongly recommend for reading. It is written by Carmine Gallo and points out how essential clear communication is in time of crisis. It also gives some advice how to do this by replacing long words with short ones, finding analogies from everyday life, personalize your massage as well as reduce your instructions to three (the rule of three).
"Like a virus, words are infectious. They can instill fear and panic or facilitate understanding and calm. Above all, they can spark action. So choose them carefully."