Find out, if possible, what level of English the foreigner who will read the lesson plan has. This will help you write the lesson plan to her level. If you can't find out what type of English the teacher speaks, you'll have to make general accommodations for her.
Write the lesson plan in basic English. Don't use any fancy words. Stay away from slang or words that might mean something different in another context.
Write short sentences. These are usually easier to decipher. A good, short sentence has a subject, a verb and an object. For instance, "Give students the papers" is a basic sentence that someone can easily follow.
Use illustrations if you can, or if they pertain to the lesson in any way. This will help people who don't speak much English understand what information you are trying to give them on the English lesson plan.