Just follow the signs at Waterloo East for Waterloo. Not for the exit. It's a short escalator down. Turn right at the bottom and exit Waterloo at the main entrance, which is (almost) at street level.
Or there are two exits which are at street level, one opposite the suburban platforms 1 - 5 into the cab rank and one opposite platform 7-ish to the bike racks. Go down the hill, straight over the pedestrian crossing, underneath the underpass past the Waterloo Tap then right around the Festival Hall onto the South Bank. It's worth walking the last bit to avoid conflict with drunk pedestrians.
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@rb58 you probably know all of that, but some people might not.)
From Victoria (for the minority of riders coming from south-east London), Fridays Central new edition is probably a simpler ride than old edition. Just turn right out of the station, keep going along Victoria Street until you get to Parliament then turn right over Westminster Bridge. When you get to the pedestrian crossing, hoick up onto the pavement to go behind City Hall, nod to the ghost of the GLC and follow Belvedere Road until it becomes the back door of the Festival Hall.
Have a lovely time everyone - I'm with you in spirit.