Start from the person sitting on the most front place at your left, and slide your gaze towards back (slightly towards middle), and continue as writing M with your gaze in the audience (in a way that you end up in the most front sitting person on your right side) – after that, look into the person sitting in the most back on the right, and follow imaginary letter W in your audience (finishing on the person sitting most in the back on your left side) – of course, the idea is not to stick to this too strictly and lose being natural, but just to have sort of a helpful pattern in order to “cover” most of your audience in the sense of eye contact.
2. Splitting the audience into squares:
You can simply draw a couple of imaginary vertical and horizontal lines over your audience, so you split them into imaginary squares (depending on the size of the audience, number of squares can be 4, 9, 16 etc), and keep the same amount of the eye contact within each of those squares, sliding your eyes from one square to another.
Passing your eyes from one person to another is most natural and smooth if you deliver one meaningful chunk of information while observing one person, and after that pass your eyes to another person (or group of persons) and deliver next meaningful chunk of information. If looking straight into people eyes is stressing you and affects your concentration, you can look into their “third eye” – the place between the eyebrows. They will feel as if you are looking into their eyes, but you can avoid their “scary” gaze.