This was posted at My Webpage by a guy names Solo MT. I belive it coverse everything but I havent tested it yet, So without further ado :

you need to declare them in your custom startpos file like this

DeclareFaction:: FN_REBEL
DeclareFaction:: FN_ALMOHAD
DeclareFaction:: FN_BYZANTINE
DeclareFaction:: FN_DANISH
DeclareFaction:: FN_EGYPTIAN
DeclareFaction:: FN_ENGLISH
DeclareFaction:: FN_FRENCH
DeclareFaction:: FN_GERMAN_HRE
DeclareFaction:: FN_ITALIAN
DeclareFaction:: FN_POLISH
DeclareFaction:: FN_RUSSIAN
DeclareFaction:: FN_SPANISH
DeclareFaction:: FN_TURKISH
DeclareFaction:: FN_ARAGONESE
DeclareFaction:: FN_BURGUNDIAN
DeclareFaction:: FN_GOLDEN_HORDE
DeclareFaction:: FN_HUNGARIAN
DeclareFaction:: FN_NOVGOROD
DeclareFaction:: FN_PAPIST
DeclareFaction:: FN_SICILIAN
DeclareFaction:: FN_SWISS
DeclareFaction:: FN_SCOTS
DeclareFaction:: FN_WELSH
DeclareFaction:: FN_IRISH
DeclareFaction:: FN_VALENCIAN
DeclareFaction:: FN_NORWEGIAN
DeclareFaction:: FN_GENOESE
DeclareFaction:: FN_CUMAN
DeclareFaction:: FN_NAVARRE
DeclareFaction:: FN_BOHEMIAN

then faction musics

then active factions like this

//========================================
//Active Factions
//which factions are to appear in the game
//and which of these factions are playable
//========================================
SetActiveFaction:: FN_ALMOHAD FT_MAJOR
SetActiveFaction:: FN_BYZANTINE FT_MAJOR
SetActiveFaction:: FN_DANISH FT_MAJOR
SetActiveFaction:: FN_EGYPTIAN FT_MAJOR
SetActiveFaction:: FN_ENGLISH FT_MAJOR
SetActiveFaction:: FN_FRENCH FT_MAJOR
SetActiveFaction:: FN_GERMAN_HRE FT_MAJOR
SetActiveFaction:: FN_ITALIAN FT_MAJOR
SetActiveFaction:: FN_POLISH FT_MAJOR
SetActiveFaction:: FN_RUSSIAN FT_MAJOR
SetActiveFaction:: FN_SPANISH FT_MAJOR
SetActiveFaction:: FN_TURKISH FT_MAJOR

SetActiveFaction:: FN_ARAGONESE FT_MAJOR
SetActiveFaction:: FN_HUNGARIAN FT_MAJOR
SetActiveFaction:: FN_SICILIAN FT_MAJOR

SetActiveFaction:: FN_PAPIST FT_MINOR
SetActiveFaction:: FN_SCOTS FT_MAJOR
SetActiveFaction:: FN_WELSH FT_MAJOR
SetActiveFaction:: FN_IRISH FT_MAJOR
SetActiveFaction:: FN_VALENCIAN FT_MAJOR
SetActiveFaction:: FN_NORWEGIAN FT_MAJOR
SetActiveFaction:: FN_GENOESE FT_MAJOR
SetActiveFaction:: FN_CUMAN FT_MAJOR
SetActiveFaction:: FN_NAVARRE FT_MAJOR
SetActiveFaction:: FN_BOHEMIAN FT_MAJOR

treasury, colour, behaviour, shields files, cultures, offices, these ones are simple to complete

be carefull about : makebuilding, makeunit , these ones crash your campaign if you set them uncorrectly (ex : catholic unit created in a pagan territory), what i do is delete them all (got no building and only king unit at start of game for tests) at first and redo them all when mod is working

new factions cannot use these textures/men directories in your unit prod file (make the game crash at battle screen) : mshelm, chainhelm, mknight,peasant, HlPlArSH, plates
disable new factions for units using these textures and/or rename folder to custom0, custom1 etc and replace texture name in unitprod file by this copy (make the shield uncoloured, but still ok for a lot of these units like militia, peasants etc)

ex : highland clans men who dont have shield

HighlandClansman INFANTRY 100 3 1 0 16 60 1 DISCOURAGED ID_SCOTLAND { CASTLE} ATTACKER,AMBUSH 2 ALL_PERIODS YES HEIGHT( 85 ), RADIUS( 25 ), SCALE( 120 ), PROJECTILE_TYPE( NONE ), SAMURAI( NO ), MARCH_SPEED( 6 ), RUN_SPEED( 12 ), CHARGE_SPEED( 13 ), MIN_TURNSPEED( 2 ), MAX_TURNSPEED( 8 ), TURN_TO_MOTION_SPEED( 8 ), MAX_INMOTION_TURN( 96 ), FORMATION_WIDTH_SPACING( 75 ), FORMATION_LENGTH_SPACING( 75 ), ENGAGEMENT_THRESHOLD( 1500 ), CHARGE_BONUS( 6 ), MELEE_BONUS( 5 ), DEFENCE_BONUS( -3 ), ARMOUR_LEVEL( 1 ), HONOUR_LEVEL( 0 ), AMMO( 0 ), FORMATIONS_PREFERRED_NUM_ROWS( 4 ) NO own_unit_tooltip1, own_unit_tooltip2, other_unit_tooltip1, other_unit_tooltip2 CUSTOM5, YES, YES ALL_FACTIONS ID_SCOTLAND Missile, Shocktroop, Spear, Cavalry SKIRMISH(0), ADVANCE_PARTY(1),
OUTFLANKING_FORCE(0), MAIN_BODY(2), HOLD_TERRAIN(0), RESERVE(1), REINFORCEMENT(1), ASSAULT(0), ASSAULT_COVER(0), ASSAULT_COVER_CASTLE_ATTACK(2), COVER(0) CATHOLIC, ORTHODOX 0 UNCONTROLLED UNFORMED SMALL YES 1 NO 0 0 PAGAN(0), CATHOLIC(0), ORTHODOX(0), ISLAMIC(0), JEWISH(0), HERETIC(0) PAGAN(0), CATHOLIC(0), ORTHODOX(0), ISLAMIC(0), JEWISH(0), HERETIC(0) SWORD YES ALL_PERIODS FN_SCOTS

see the CUSTOM5, YES, YES where custom5 is a copy of peasant directory

nb : if you use FN_newfactionname in you prod file then you must have the declaration in all your startpos files (ive moved them all)

you also need to edit your loc file

default_faction_specific entries, should keep the order from your startpos file
@{Rebelles}
@{Almohades}
@{Byzance}
@{Danemark}
@{Egypte}
@{Angletterre}
@{France}
@{Germanique}
@{Venise}
@{Pologne}
@{Russie}
@{Castille}
@{Turquie}
@{Aragon}
@{Bourgogne}
@{Horde}
@{Hongrie}
@{Novgorod}
@{Papauté}
@{Sicile}
@{Suisse}
@{Ecosse}
@{Galles}
@{Irlande}
@{Valence}
@{Norvège}
@{Gènes}
@{Coumans}
@{Navarre}
@{Bohême}

well sorry its in french :p

there are entries in names.txt, changes.txt and default region specific.txt i think too.

i may have forgot one or two things tho but i made my mod work with new factions,buildings so i can correct this if need some details