Talk directly to the person, maintain eye contact, speak using simple sentences avoiding jargon and complex terminology. Determine what he or she wants, and determine how best to deliver it by consulting with the person. This may sound very simple, and it is.
Appropriate interaction is common courtesy and common sense.
Such behaviours are not generally intended to give offence. Maintaining eye contact shows respect and courtesy to the person you are interacting with. It shows you are listening and trying to help. This is a good rule to follow with almost anybody, but is especially important when dealing with people with intellectual disabilities whose receptive language skills are typically low. People respond best to their first names and people with intellectual disabilities are accustomed to being addressed by their first names.