The detailed rules depend on exactly how long exactly what flavour of rating on exactly which type of licence have been expired, and seem to change every five minutes anyway.
But they pretty well all amount to "training as necessary to pass a test, followed by passing a test". The "training as necessary" in some cases may be nothing or just a practice test, in my case, after a thirteen year lapse, it did consist of going through the whole of the PPL training again, but in half the time.