My yellow sets are now grown to 18 inches tall and I was told that I should step on the plant .?

You don't have to step on it, just break over the green top at least 2/3 of the height. It is supposed to keep the onion plant from trying to make flower/seeds and concentrate on making the onion bulb bigger.

That sounds rather strange advice, however, it does make sense if that's what they like. I think you should call your local garden centre to confirm it.