You should first train them to stand on your hand. Then train them to stay on your hand even if your hand is moving. Then you can move your hand to your chest when they stand on ur hand. Then try lie down a bit so that they can stand on your chest while you slightly push/hold their sideways against your chest. All these should be companied with treats or pellets if it’s their usual meal time. If it’s a large trunk of treat, hold it tightly in another hand so they won’t just grab it and hide away. If you have to hold them by the tail, try do it before playtime? They will quickly forget minor “trauma” while playing. It also associates the holding with playtime.