From my days as a guard on the Sittingbourne and Kemsley Light Railway:
1) The driver obtains the token (or ticket) for the section ahead. Not normally needed in mainline practice today as I understand.
2) The driver shows the token/ticket to the guard.
3) The starting signal is cleared¹. This does NOT permit the driver to start, merely tells him that the section ahead is clear.
4) The "Right away" is given by the station staff TO THE GUARD. As Stewie says, it may be a plunger, a hand signal (face the guard, whistle and raise your hand above your head) or by a paddle with a red face and a green or white face.
5) When the guard is happy, then he signals to the driver, traditionally by green flag, today by buzzer/bell.
6) Now the driver can start and drive to the next scheduled stop, stop signal, or signal from the guard (though the latter is only for abnormal circumstances of course).
Heritage lines may follow an older style, but must comply with rulebooks approved by the inspectorate. One job before being passed as a guard is to learn the rulebook and be tested on it, just like main line. Mainline practice uses computerised signalling and some of the steps above may be combined.
It may seem strange that both a token and a signal are needed, but they demonstrate slightly different things. The token shows that the loco is the only one permitted in the block whereas the signal shows that the block is physically clear, for instance crossing gates are locked open for the train.
¹This may not apply on certain short lines where there is only one block.
