Use visual aids, gestures, pointing, write important words or dates. These will not guarantee comprehension but you will increase the chances that the deaf person will understand you. If you cannot understand a deaf person's speech, offer some paper and a pen to encourage writing notes instead of or in addition to talking. Writing notes will work well with some deaf people and not with others. Because of that, reading and writing may be more difficult and you may need to adjust how you write.
Some deaf people have no difficulties.
Begin by writing as you ordinarily would do. If you notice the deaf person responds inappropriately, follow the guidelines below. Keep it short and simple. You do not need to write in full sentences. Draw diagrams, charts, etc.