With video, attention is critical. The longer a clip, the less likely the audience (your students) will stay involved. Short videos reinforce concepts and allow more practice more often.
How long should an instructional video be? Only as long as it needs to be and no longer i.e. short.
Students can listen to you lecture and see diagrams of concepts to begin, but a narrated demonstration provides an immersive experience that you can elaborate on as students witness events.
Identifying others in the field of your subject who can give concepts a contextual meaning. Being able to guide an interview to indicate to students the importance of specific elements can be extremely useful.
Watching software in action is often how students learn to use different products. With your own demonstrations, you can illustrate what tools are important to learn and also focus on the concepts key to your course. Create short videos of screen captures to help with difficult concepts when dealing with software.
Illustrate diagrams and formula with a pen and paper to provide a more immersive experience for online students. Using a personal level of narration also helps students feel as part of the learning community as they identify with working form pen and paper themselves.