Hand rearing can be very rewarding but it can also be very distressing when things go wrong. In our experience most problems occur due to poor hygiene or not using a probiotic (especially with incubator hatched chicks).
Avisafe (EnviroClens outside Europe) is also ideal for keeping utensils and equipment clean.
For instructions on how to use a crop tube please see the below link:
Finally we also have a page with recommended brooder temperatures. A useful reference guide if things are not quite going right.