Questions about Client side prediction

I understand how client side prediction works. I have some questions which I need to clarify:

1. What exactly do we send in the userCmd? In my case, what are the information I should be sending for a fps shooter?
2. When do we send it? In the main animation loop of the client? 
3. If the server runs the game at 60 fps and the client runs the game at 30 fps, the client will always snap back even after server replaying the stored commands. To fix this we will need to update the game based on deltaTime right?
4. If the …