Is this a good way to make omelette. How can I learn to me it?

I'd LOVE to learn how to make omelette. I used to work in a restaurant and in the morning we used to make omelettes with onions, bacon, and tomato. The eggs were already made, so first I put the oil then the ingredients we let it cook and then we put the eggs mix and flip it around. Ive seen some videos and they always put the eggs first.. is there any difference?? Also whats best to put: oil, melted butter or spray?? Thank you!