Being conscious of writing to someone will make your writing more relaxed and natural. You can get valuable feedback about readers by asking a friend, family member, or colleague about their special interests and prior knowledge. You must win your audience's trust. Don't write like someone you're not. At the same time, don't be afraid to challenge your audience, to make them reach a bit.
Most good writers shake their readers up occasionally.
Be polite and professional, but never servile or arrogant. We all have a tendency to try to impress our audience. Fight this tendency. It generally leads to pretentious diction, obscurity, and even dishonesty, and you will quickly lose credibility as a writer.