Post your schedule, read it, and refer to it often. If you want to vary it, give lots of warning and preparation. Announce in advance what is going to happen, then give repeated reminders as the time approaches. When giving directions, announce what you are going to say first, then say it, and repeat what you have said. If you are teaching something new, break down large tasks into small tasks.
Large tasks can quickly overwhelm the child.
By breaking the task down into manageable parts, so that each component appears small enough to be doable, the child can sidestep the emotion of being overwhelmed. Be clear and direct about your expectations, don't assume a child with ADD can understand subtle messages, but don't assume they can't, either. A glance or touch can retrieve a child from a daydream, give permission to ask a question, or provide silent reassurance. Pay attention to connectedness. Children with ADD need to feel engaged and connected.