If a kitten is stuck in the birth canal, then the process for trying to assist is as follows:
1: Get hold of the kitten using a clean towel.
2: Gently but firmly pull the kitten out in a slightly downward angle. If you can feel movement, simply keep pulling at the same pace until the kitten is delivered.
3: If it is still stuck or the mother makes it impossible for you to help, then a vet will be the only real option as this may require professional intervention.