The first step is to face the person you are communicating with in order to get his/her attention and show him/her that you are talking to them. Someone with an impaired hearing needs to have an eye contact with you for successful communication.
You need to speak slowly and clearly so that every word can be understood by the impaired person. However, do keep in mind not to shout - maintaining your natural speech pattern is advised.
Do not make distracting movements and keep your hands away from your face as this will distract someone with a hearing impairment.
The important thing is to stay patient when communicating with physically challenged people. They might not immediately comprehend what you are trying to convey to them and vice versa.
When taking help from a third person, you have to keep your attention towards the physically impaired person by keeping eye contact while the third person becomes a buffer in the communication process. You have to make the impaired person feel as if you are talking to him/her, so direct your face towards him/her as you speak, and let the third person continue the interpretation process.
The communication needs to take place in a quiet location, where it will be easier for the impaired person to understand you without the background noise causing any distractions. Remember, that some people with autism could be uncomfortable around too many people or with too many distractions in the background, so you do not need to put them in this situation and instead communicate in an environment which is comfortable for them.