Loki is getting used to being placed on her back now but Rocko is biting while hes on his back hard bites so I have to hold his head

You could always put your hand on his back while he's standing upright, and work toward gradually tilting your hand more and more until he's on his back. You could reward him for tilting a little bit without biting, then for tilting more.

This video actually taught me how to teach my conure to lie on its back: https://youtu.be/32MVDxH27T8

Cockatiels are very different though. I wouldn't get your hopes up. Cockatiels aren't popular for lying on their backs. Most hate it.

Thats what I was doing I can get him on his back and hes smart he will realise what im trying to do.

Loki dosent make a fuss at all she now just lets me place her on her back maybe she will lie down without my hand some day and im sure Rocko will be doing it soon.Ill watch the video now.Thanks.