One other tip when inserting a feeding tube. Straighten and extend the neck of the chicken and the tube slides right in. Once the tube is in, I hold both the chicken's beak open and the tube with my weak hand while syringing with the strong hand.
If the chicken struggles during the process, keep calm, don't yell, exert firm control over the squirming chicken, then when she calms down, resume tube feeding. Once the chicken knows that struggling isn't going to get her anywhere, she usually calms down, especially after she feels warm food and liquid going into her crop.
With practice, you develop confidence and your chicken will be much calmer when you are confident and calm.