Ensuring you have the best commanders can win many battles, even those where your kingdom is heavily outnumbered. There are several ways to upgrade regular courtiers to esteemed commanders, listed below.
From their Youth: A military education.
A military education can greatly increase a character's martial skill, even if they get traits such as the misguided warrior or the tough soldier. If a character has these traits, they can still become great generals. Any character (including the player) can level up their military education by fighting in battles. A scenario will occur where the character will learn more about leading troops, and will upgrade their martial skill to the next level. Even misguided warriors can work their way up to brilliant strategists, which extremely useful in combat situations. Also, if a character already has a military trait, they can gain certain leader bonuses, which influence the battle greatly. If a character has a low martial education, they can still work their way up on the battlefield.
Traits gained in combat:
Placing the player or other characters on he battlefield can help them gain traits. After fighting a battle, character may gain trait regarding a martial gain, such as brave, patient, or a base skill of martial upgrade by 1. The zealous trait can be gained when fighting enemies of another religion, and the wroth trait can also be gained.
Crusades can be a great blessing to any commander. Being in the territory that the crusade was called on gives the crusader trait, which is helpful, it also gives positive two in martial, as well as vassal opinion boosts, as well as anyone with the same trait. It is advisable to, even if the player has no intent on leading troops, to take command of the army in the crusade, as well as their heir and all of their commanders, to help boost their martial. It may help to place extra courtiers with a decent skill on the field as well, in case of the death of one of the player's commanders.
Protecting your Commanders:
The longer character serve as commanders, the more martial and military skill they will gain, but this also comes with a risk. Never place a commander with the wounded or ill trait at the head of an army, because they will lose personal combat skill and will be more vulnerable to being killed on the battle. Extended combat can also cause characters to gain less desirable traits, such as maimed, craven, or incapable. To help protect your commanders, make sure they have a positive personal combat skill, and keep wounded, sick, or old commanders off the field. Commanders can also be captured and imprisoned, which may be difficult to return. Finally, although age does not affect commander's martial skills too much, it is advisable to relieve commanders after they reach their 60's. Using these grizzled veterans to tutor children in your realms will ensure new commanders will keep arriving, and placing them on the battle at a young age can enhance them to be truly lethal in their later years.
Keep injured, sick, or old commanders off the field unless absolutely necessary.
Training child courtiers or your own children with older commanders can ensure new generals with skill. Note: do not train children with commanders that are too old or are likely to die before they finish the education, it lowers the chance of the child becoming a good commander greatly.
Keeping them on the battle field can be hazardous, but can reduce risk with high martial skill.
Use weaker commanders on unimportant battle, with the hope they will increase their martial ability.
Never use a commander with a martial skill of less than 10, as this will actually lower your army's effectiveness. (unless it is a guaranteed win, and you are trying to upgrade the commander's education or skill.)
by Exotic Lemon