You just need to put what units you want in the settlements in descr_strat, below is an example for a rebel settlement near byz empire, which are all listed at the end of descr_strat:

character sub_faction byzantium, Georgios Iasites, general, male, age 29, x 191, y 71
army
unit Byzantine Spearmen exp 0 armour 0 weapon_lvl 0
unit Trebizond Archers exp 1 armour 0 weapon_lvl 0
unit Byzantine Spearmen exp 0 armour 0 weapon_lvl 0
unit Trebizond Archers exp 1 armour 0 weapon_lvl 0

You just need the coordinates of the settlement by using show_cursorstat. Works the same for faction settlements, except you put that information in the factions area of desc_strat, and the difference being you don't use sub_faction, example below:

character Nero Corrigarius, general, male, age 36, x 147, y 90
traits GoodCommander 1 , NaturalMilitarySkill 1 , PublicFaith 2 , GoodAdministrator 1 , Intelligent 1
army
unit Pavise Crossbowmen exp 1 armour 0 weapon_lvl 0
unit Mailed Knights exp 0 armour 0 weapon_lvl 0
unit Italian Spear Militia exp 1 armour 0 weapon_lvl 0
unit Italian Spear Militia exp 1 armour 0 weapon_lvl 0

Hope that helps
Nem