The cornerman system (or 'corner marker system') works pretty well for larger groups, and those with some slow and some fast riders; it encourages overtaking. If there's only three or four riders, or everyone rides at about the same pace, follow the leader is normally a better match.
Most forums try to explain the cornerman system but make it sound far more complex than it is.
In short, there is a 'leader' and a 'tail' (who might also be called a 'tail-end Charlie', 'TEC', 'last man' or 'tailgunner'), and everyone should be able to identify the tail from the front and the leader from behind.
The leader goes at the front of the ride and knows where they're going, the tail stays at the back - nobody overtakes the leader, and the tail overtakes nobody.
Whenever the ride does anything other than go straight on, the rider immediately behind the leader stops and marks the corner - they are now a 'cornerman' or a 'cornermarker'. If the leader thinks a marker's needed somewhere then they'll point to where the marker should be, and the next rider should stop and mark whatever's been pointed at. Marking a corner is exactly that - pulling over (often just to the side of the road, but if there's a pavement or something that's fine too) so as to be able to direct other riders.
Riders approaching the corner will see this rider and know to turn, or at least that they need to do something other than just carry on riding straight on. It is quite important that the cornermarker positions themselves such that they are obvious to oncoming riders (not hidden behind a sign, or already round the corner), and also such that it is obvious what the oncoming rider must do - which turning, roundabout exit or lane they should be taking.
As the tail approaches the corner, the cornermarker gets back on their bike and rejoins the ride - pulling in before the tail, but after the previous rider has taken the corner.
And that's basically all there is to it. During the ride the faster riders will naturaly find themselves overtaking lots, and therefore at the front a lot, and so marking corners. Slower riders will sit in the middle with a steady stream of corner markers guiding them, and faster riders overtaking them to mark more corners.