If you want a quick version:
Attackers get X points (depending on game size)
Defenders receive 60% of the above
Convoy deploys on one map short edge and exits from the other. Attacker can move on from either long edge in any movement phase.
Defenders gain victory points for destroying/capturing attackers as normal and also get the full points value of their defended ships if they can withdraw off the other end of the table
Attackers gain victory points for destroying/capturing defenders as normal but gain a set of nice, fragile targets in the form of the convoy ships which are also worth points, and can withdraw freely from the long table edges.
If you want a quick stand-in, for every 200 points in the defending fleet, say they get a transport that behaves exactly like a galliot except that it has neither troops, guns or grapnels. It's worth the same as a galliot because it's stuffed to the gunwhales with valuable purloinables.
So, a 1,000 point defending fleet has to protect 5 transports against 600 points of attackers.
I heartily approve of convoy duty coming back into the game, at least in part because it's what raiders are supposed to do - currently they just get it in the neck because they (unsurprisingly) can't go up a 'proper' warship.