This guide explains how to actually level the Roguery skill in the Early Access version of the game, to get those delicious Looting+ Perks.
How to Level Up
Now hold up just a second. How the hell are Peasants going to help me level up the Roguery skill? The Game Help Text says raiding caravans, but nothing about peasants!
Well, you see in some twist of Irony, you're not attacking peasants, but Rescuing them. You'll need a lot of them (20+). On top of that, you'll also have to rescue them from a bandit group ideally looters, around ten of which you should be able to solo right out the gate, holy rock to the head notwithstanding.
So find yourself some looters with prisoners (Hold the Alt key on the map to see who's got Wounded (+W#) and Prisoners (+P#).
Once you find a party you can engage with prisoners, confirm they are a Peasant class prisoner, (or, one of the upgrades, Watchman or Mercenary Guard for example, though that would be extraordinarily rare). Peasant works, from Kuzhait to Vlandian, but ideally you want them all the be the same culture because of how troop XP works.
Once you rescue them, we can move onto stage 2, the hardest. Try to get around 20-40 Peasants, to absorb losses.
So now you've got your peasants? What next? Well, now it's time to grind XP. You see, the Humble Peasant upgrades into the poor T1 troop "Watchman", but he in turn upgrades into one of two options. One of these is what we need to get your Roguery training going.
So train those peasants! Pick only looters, and make sure (If you're not doing auto resolves) to have only the peasants get the kills to speed up XP (Yes this works). Fortunately, this isn't very dangerous for the peasants or the looters, as they will mostly just huck rocks at one another, with a few going in with pitchforks. As long as you aren't grossly outnumbered (Try to go 1:1 or less to be safe) it should be pretty easy. If things start turning against you, you can always kill tons of looters yourself.
Your peasants will level up to Watchmen quite fast, Watchmen have no ranged attack, so they're slightly more dangerous to level up against looters, so expect more fatalities. As a tip, try to upgrade all your troops at once. Because of how troop XP, the entire unit class shares an XP pool. So don't upgrade any until all are ready.
Repeat the process with your Watchmen and upgrade them to Mercenary Guards, this is what we want.
Non-Lethal Firing Squad
Now that you have your Mercenary Guards (20+ is ideal, and this can take on groups of looters over five times their size) it's just a matter of grinding more battles.
You see, the Mercenary guard have crossbows that fire Blunt bolts. These bolts are non-lethal and KO the target instead of kill. Meaning you will have a 100% Prisoner capture rate if only these guys do your killing, and you then ransom off entire armies for massive Roguery XP. Have fun.
As a final parting tip, make sure you don't ransom units from the World Map, you have to actually go into the Random menu (It looks like the party menu) and then you can ransom all. If you ransom from the world map quick option, you only get XP for one prisoner, even if you dump them all.
XP seems to be per prisoner not tied to Gold value.