This guide is intended to help you where it is actually difficult: At the start of the game, when you have a small band and can’t afford to claim most strongholds.
Get Labour to 120
This is absolutely essential. When your Labour skill is at least 120, you get the ability Stamina, which doubles your manpower. It does this doubling after any other modifiers to manpower, so it is huge and absolutely essential. Basically every character should have at least 120 Labour.
Chaotic: Build to Fight
Chaotic characters get their second-highest skill score as manpower.
This means that they will never be the absolute best at manpower—because they can’t put everything into manpower. But in practice they are often the most useful, because you don’t need to focus so much on manpower; other than getting Labour to 120, all you need to do to make a chaotic character good at manpower is give them two strong skills, which you probably were going to do anyway: Perhaps Fight and Guard for a fighter type, or Hunt and Dodge for a rogue type, or Blast and Ward for a wizard type.
As long as you don’t put everything into one skill—which is generally a bad build, even though the AI does it a lot—you’ll be fine.
Lawful: Get Some Utility
Lawful characters contribute their highest Utility skill (i.e. Labour, Thwart, or Weave) as manpower.
This means that you can build a lawful character with a decent combat build and skill get a lot of manpower as long as you put some skill points into Utility. E.g. if you’ve been building a tank by putting a lot into Guard and some into Fight, just put some into Labour; or if you’ve been building a DPS wizard by putting a lot into Blast and some into Ward, put some into Weave.
Bato’s default build is a good example of how to do this.
Neutral: Start from Scratch
Neutral characters contribute their Utility stat squared. It doesn’t matter what their Labour, Thwart, or Weave skill is; it only matters what their Utility stat is.
Theoretically, this means that neutral characters can have the most manpower, because they can split between Labour, Thwart, and Weave and get all the high-level skills that boost manpower.
In practice, it means that almost every neutral character you find will have a pathetic amount of manpower.
Why? Because the AI builds characters for combat, not manpower. So typically they will sink almost none of their skill points into Utility. Sometimes literally none: I have gotten several neutral characters who were like level 20 and still only had 1 Utility—which means they only produce 1 manpower.
This makes neutral characters (ironically) wildly unbalanced; they will either be useless at manpower, or amazing at it. This is how you get them to be amazing.
Take your neutral characters to the Fundaments Shrine, separate them from your party, and have them each touch the altar in the back that talks about “Renewing yourself”. Pay 1000 gp and that will reset the character back to level 1.
From there, go level the character (it’ll happen fast as long as they don’t go down), and put lots and lots of points into Utility skills. In practice what I generally do is give them a few points of Attack and Defend on the same row (e.g. Fight and Guard, or Blast and Ward), and then from then on, put everything—everything—into the corresponding Utility skill (e.g. Labour and Weave respectively). The result will be a character who is lousy at combat, but truly outstanding at manpower.