General question about declaring functions

#1

suppose i have more than one functions in a code named f1 , f2, f3,
now, should i first declare their name on top like-
void f1
void f2
void f3
…
and then write their coidng? or else should i write their declaration along with their coding? which is accepted? if both are accepted then which one is better?

#2

both will accepted.
it is preferred to define a function while declaring.
but when we are using an other function inside the first function then we have to declare other function before