It's as easy as putting units in in fact it's quite the same until you come to the CLsystem. (How to simply add a building should be plenty of info in the forum it's just the Lsystem that is a bit tricky).
You basically use these files:
CIV4ArtDefines_Building.xml
example:
<BuildingArtInfo>
<Type>
ART_DEF_BUILDING_ACADEMY</Type>
<bAnimated>0</bAnimated>
<fScale>0.9</fScale>
<fInterfaceScale>1.5</fInterfaceScale>
<NIF>Art/Structures/Buildings/Academy/Academy.nif</NIF>
<KFM/>
<Button>,Art/Interface/Buttons/Buildings/Academy.dds,Art/Interface/Buttons/Buildings_Atlas.dds,7,11</Button>
</BuildingArtInfo>
CIV4BuildingClassInfos.xml
<BuildingClassInfo>
<Type>
BUILDINGCLASS_ACADEMY</Type>
<Description>TXT_KEY_BUILDING_ACADEMY</Description>
<iMaxGlobalInstances>-1</iMaxGlobalInstances>
<iMaxTeamInstances>-1</iMaxTeamInstances>
<iMaxPlayerInstances>-1</iMaxPlayerInstances>
<iExtraPlayerInstances>0</iExtraPlayerInstances>
<bNoLimit>0</bNoLimit>
<bMonument>0</bMonument>
<DefaultBuilding>
BUILDING_ACADEMY</DefaultBuilding>
<VictoryThresholds/>
</BuildingClassInfo>
CIV4BuildingInfos.xml
<BuildingInfo>
<BuildingClass>
BUILDINGCLASS_ACADEMY</BuildingClass>
<Type>
BUILDING_ACADEMY</Type>
<SpecialBuildingType>NONE</SpecialBuildingType>
<Description>TXT_KEY_BUILDING_ACADEMY</Description>
<Civilopedia>TXT_KEY_BUILDING_ACADEMY_PEDIA</Civilopedia>
<Strategy>TXT_KEY_BUILDING_ACADEMY_STRATEGY</Strategy>
<Advisor>ADVISOR_SCIENCE</Advisor>
<ArtDefineTag>ART_DEF_BUILDING_ACADEMY</ArtDefineTag>
<MovieDefineTag>NONE</MovieDefineTag>
<HolyCity>NONE</HolyCity>
<ReligionType>NONE</ReligionType>
<StateReligion>NONE</StateReligion>
<PrereqReligion>NONE</PrereqReligion>
<GlobalReligionCommerce>NONE</GlobalReligionCommerce>
<VictoryPrereq>NONE</VictoryPrereq>
<FreeStartEra>NONE</FreeStartEra>
<MaxStartEra>NONE</MaxStartEra>
<ObsoleteTech>NONE</ObsoleteTech>
<PrereqTech>NONE</PrereqTech>
<TechTypes/>
<Bonus>NONE</Bonus>
<PrereqBonuses/>
<ProductionTraits/>
<HappinessTraits/>
<PowerBonus>NONE</PowerBonus>
<FreeBonus>NONE</FreeBonus>
<iNumFreeBonuses>0</iNumFreeBonuses>
<FreeBuilding>NONE</FreeBuilding>
<FreePromotion>NONE</FreePromotion>
<CivicOption>NONE</CivicOption>
<GreatPeopleUnitClass>NONE</GreatPeopleUnitClass>
<iGreatPeopleRateChange>0</iGreatPeopleRateChange>
<iHurryAngerModifier>0</iHurryAngerModifier>
<bBorderObstacle>0</bBorderObstacle>
<bTeamShare>0</bTeamShare>
<bWater>0</bWater>
<bRiver>0</bRiver>
<bPower>0</bPower>
<bDirtyPower>0</bDirtyPower>
<bAreaCleanPower>0</bAreaCleanPower>
<bDiploVote>0</bDiploVote>
<bForceTeamVoteEligible>0</bForceTeamVoteEligible>
<bCapital>0</bCapital>
<bGovernmentCenter>0</bGovernmentCenter>
<bGoldenAge>0</bGoldenAge>
<bMapCentering>0</bMapCentering>
<bNoUnhappiness>0</bNoUnhappiness>
<bNoUnhealthyPopulation>0</bNoUnhealthyPopulation>
<bBuildingOnlyHealthy>0</bBuildingOnlyHealthy>
<bNeverCapture>0</bNeverCapture>
<bNukeImmune>0</bNukeImmune>
<bPrereqReligion>0</bPrereqReligion>
<bCenterInCity>0</bCenterInCity>
<iAIWeight>0</iAIWeight>
<iCost>-1</iCost>
<iHurryCostModifier>0</iHurryCostModifier>
<iMinAreaSize>-1</iMinAreaSize>
<iConquestProb>100</iConquestProb>
<iCitiesPrereq>0</iCitiesPrereq>
<iTeamsPrereq>0</iTeamsPrereq>
<iLevelPrereq>0</iLevelPrereq>
<iMinLatitude>0</iMinLatitude>
<iMaxLatitude>90</iMaxLatitude>
<iGreatPeopleRateModifier>0</iGreatPeopleRateModifier>
<iGreatGeneralRateModifier>0</iGreatGeneralRateModifier>
<iDomesticGreatGeneralRateModifier>0</iDomesticGreatGeneralRateModifier>
<iGlobalGreatPeopleRateModifier>0</iGlobalGreatPeopleRateModifier>
<iAnarchyModifier>0</iAnarchyModifier>
<iGlobalHurryModifier>0</iGlobalHurryModifier>
<iExperience>0</iExperience>
<iGlobalExperience>0</iGlobalExperience>
<iFoodKept>0</iFoodKept>
<iAirlift>0</iAirlift>
<iAirModifier>0</iAirModifier>
<iNukeModifier>0</iNukeModifier>
<iNukeExplosionRand>0</iNukeExplosionRand>
<iFreeSpecialist>0</iFreeSpecialist>
<iAreaFreeSpecialist>0</iAreaFreeSpecialist>
<iGlobalFreeSpecialist>0</iGlobalFreeSpecialist>
<iMaintenanceModifier>0</iMaintenanceModifier>
<iWarWearinessModifier>0</iWarWearinessModifier>
<iGlobalWarWearinessModifier>0</iGlobalWarWearinessModifier>
<iHealRateChange>0</iHealRateChange>
<iHealth>0</iHealth>
<iAreaHealth>0</iAreaHealth>
<iGlobalHealth>0</iGlobalHealth>
<iHappiness>0</iHappiness>
<iAreaHappiness>0</iAreaHappiness>
<iGlobalHappiness>0</iGlobalHappiness>
<iStateReligionHappiness>0</iStateReligionHappiness>
<iWorkerSpeedModifier>0</iWorkerSpeedModifier>
<iMilitaryProductionModifier>0</iMilitaryProductionModifier>
<iSpaceProductionModifier>0</iSpaceProductionModifier>
<iGlobalSpaceProductionModifier>0</iGlobalSpaceProductionModifier>
<iTradeRoutes>0</iTradeRoutes>
<iCoastalTradeRoutes>0</iCoastalTradeRoutes>
<iGlobalTradeRoutes>0</iGlobalTradeRoutes>
<iTradeRouteModifier>0</iTradeRouteModifier>
<iGlobalPopulationChange>0</iGlobalPopulationChange>
<iFreeTechs>0</iFreeTechs>
<iDefense>0</iDefense>
<iBombardDefense>0</iBombardDefense>
<iAllCityDefense>0</iAllCityDefense>
<iAsset>6</iAsset>
<iPower>0</iPower>
<fVisibilityPriority>1.0</fVisibilityPriority>
<SeaPlotYieldChanges/>
<GlobalSeaPlotYieldChanges/>
<YieldChanges/>
<CommerceChanges/>
<ObsoleteSafeCommerceChanges>
<iCommerce>0</iCommerce>
<iCommerce>0</iCommerce>
<iCommerce>4</iCommerce>
</ObsoleteSafeCommerceChanges>
<CommerceChangeDoubleTimes/>
<CommerceModifiers>
<iCommerce>0</iCommerce>
<iCommerce>50</iCommerce>
</CommerceModifiers>
<GlobalCommerceModifiers/>
<SpecialistExtraCommerces/>
<StateReligionCommerces/>
<CommerceHappinesses/>
<ReligionChanges/>
<SpecialistCounts/>
<FreeSpecialistCounts/>
<CommerceFlexibles/>
<CommerceChangeOriginalOwners>
<bCommerceChangeOriginalOwner>0</bCommerceChangeOriginalOwner>
<bCommerceChangeOriginalOwner>0</bCommerceChangeOriginalOwner>
<bCommerceChangeOriginalOwner>1</bCommerceChangeOriginalOwner>
</CommerceChangeOriginalOwners>
<ConstructSound>AS2D_BUILD_LIBRARY</ConstructSound>
<BonusHealthChanges/>
<BonusHappinessChanges/>
<BonusProductionModifiers/>
<UnitCombatFreeExperiences/>
<DomainFreeExperiences/>
<DomainProductionModifiers/>
<BuildingHappinessChanges/>
<PrereqBuildingClasses/>
<BuildingClassNeededs/>
<SpecialistYieldChanges/>
<BonusYieldModifiers/>
<Flavors>
<Flavor>
<FlavorType>FLAVOR_SCIENCE</FlavorType>
<iFlavor>10</iFlavor>
</Flavor>
</Flavors>
<HotKey/>
<bAltDown>0</bAltDown>
<bShiftDown>0</bShiftDown>
<bCtrlDown>0</bCtrlDown>
<iHotKeyPriority>0</iHotKeyPriority>
</BuildingInfo>
Have a look at those files and see how they link.
All lines that should concern you are in bold, the rest you can probably ignore for now
The tricky part Kael offered help for is this one:
CIV4CityLSystem.xml
<!-- LEAF 2x1 BUILDINGS -->
<ArtRef Name="building:BUILDING_ACADEMY">
<Attribute Class="Rotation">0,180</Attribute>
<Scale>2.0</Scale>
</ArtRef>
For now you could simply replace existing buildings with your new added ones in that file if they match in size but this won't lead to perfect results so learning more about it is obligatory on long term.
i think all of them have .dds textures applied to them instead of just BMP textures. i hope they work
