Since you mentioned you were moding an existing mod, I can skip the newer edit the core civ story and get to the work.
Alright, to make a new skin the first thing you have to do is make the skin .dds file.
From your posts, I assume you know how to do that.
The next step is to make a copy of the .nif file and all the other skin files (the ones you did not change) and put it in a new folder next to the old one. (you don't need to copy the animations, you can just link to them from the XML)
Step three, copy your new skin into the folder so that the .nif file when opened shows the new skin instead of the old one.
Than make a new art define linking to the new folder and the nif file within it.
Thus far, I think I have probably not said anything new to you since you have already created units. But here is where the interesting part begins.
To change how a unit looks for all civilizations, just enter CIV4UnitInfos.xml, find the unit in question and find the part named <UnitMeshGroups>.
For your marine the text should look something like this (from base civ)
Code:
<UnitMeshGroups>
<iGroupSize>3</iGroupSize>
<fMaxSpeed>1.75</fMaxSpeed>
<fPadTime>1</fPadTime>
<iMeleeWaveSize>3</iMeleeWaveSize>
<iRangedWaveSize>0</iRangedWaveSize>
<UnitMeshGroup>
<iRequired>3</iRequired>
<EarlyArtDefineTag>[COLOR="Red"]ART_DEF_UNIT_MARINE[/COLOR]</EarlyArtDefineTag>
</UnitMeshGroup>
</UnitMeshGroups>
See the bit I colored red? Just delete that and write in it's place the name of your new art define you created just moments ago.
And there you have it, all civilizations will now use the new art instead of the old one.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Section 2: Flavor
So you want to add flavor (different skins or even models for different civilizations).
This is a bit harder but not significantly so. You will again have to make a new art define for each skin you want to use. So each different skin means one extra folder, one extra copy of the nif file and one extra skin.
After that is done, you have to connect the art to the units. To do this, open the file named: CIV4CivilizationInfos.xml
Inside it, find the civilization you want to edit.
Once you find the civ, find a tag titled: <UnitArtStyleType>
The contents of this tag define an art style that the units of that civilization will use.
Now, depending on the type of mod you are working with, the text in the tag can be just about anything. In base civ, the values are shared for multiple civilizations. While mods like Varies Delectat might have separate tags for each civ.
Subsection 1.
In case of 1 art style for 1 civ, all you have to do is open the file called CIV4UnitArtStyleTypeInfos.xml, find the art style that correspond to the name given in the CIV4CivilizationInfos.xml file.
After that is done, go to the end of the art style (where the <StyleUnits> tag closes) and insert this code before the closing line:
Code:
<StyleUnit>
<UnitType>[COLOR="red"]The name of the unit you want to change the art for. This can be read from the CIV4UnitInfos.xml file.[/COLOR]</UnitType>
<UnitMeshGroup> [COLOR="red"]This is the mesh group. Your marine only has one, but if you are working with units that have multiple (like the settler) make 1 of these tags for each mesh group.[/COLOR]
<EarlyArtDefineTag>[COLOR="red"]The new art define you want[/COLOR]</EarlyArtDefineTag>
<LateArtDefineTag>[COLOR="red"]The new art define you want[/COLOR]</LateArtDefineTag>
<MiddleArtDefineTag>[COLOR="red"]The new art define you want[/COLOR]</MiddleArtDefineTag>
</UnitMeshGroup>
</StyleUnit>
Also, keep in mind that an art style might already contain a change to the unit.
In case this is so it will be clearly visible in that one of the tags will already have the unit name you desire. For example, you might run into something like this:
Code:
<StyleUnit>
<UnitType>[COLOR="Red"]UNIT_MARINE[/COLOR]</UnitType>
<UnitMeshGroup>
<EarlyArtDefineTag>[COLOR="Blue"]ART_DEF_UNIT_MARINE_1[/COLOR]</EarlyArtDefineTag>
<LateArtDefineTag>[COLOR="blue"]ART_DEF_UNIT_MARINE_1[/COLOR]</LateArtDefineTag>
<MiddleArtDefineTag>[COLOR="blue"]ART_DEF_UNIT_MARINE_1[/COLOR]</MiddleArtDefineTag>
</UnitMeshGroup>
</StyleUnit>
If you do, than there is no need to make a new copy of it in the end like I said. Instead, just edit the part I colored blue to match the art style you want.
Subsection 2:
Now, if you are working with a mod that does not have specific art styles for each civilization. (you can check this in the CIV4CivilizationInfos.xml file. To do this, simply check if more than one civ uses the same value for the <UnitArtStyleType> tag.
If this is the case, you will need to make a new art style for your civ. That is fairly easy.
Again, open the CIV4UnitArtStyleTypeInfos.xml file and find the art style the civ uses.
Make a copy of the entire art style (bordered for your convenience by the <UnitArtStyleTypeInfo> tags.) Paste this copy near the end of the file just before the <UnitArtStyleTypeInfos> tag closes.
Change the value of the <Type> tag in the copied style.
For example:
Code:
<Type>UNIT_ARTSTYLE_ASIAN</Type>
becomes:
Code:
<Type>UNIT_ARTSTYLE_MY_NEW_STYLE</Type>
After that is done, do the same to the new art style that you would have done in Section 2 - Subsection 1.
Finally open the CIV4CivilizationInfos.xml file, find your chosen civilisation and change the value of the <ArtStyleType> tag to match your new name.
For example:
Code:
<UnitArtStyleType>UNIT_ARTSTYLE_ASIAN</UnitArtStyleType>
Becomes:
Code:
<UnitArtStyleType>UNIT_ARTSTYLE_MY_NEW_STYLE</UnitArtStyleType>
Once that is done, it should all work fine in game.
Try it out, and if something fails come back here with more questions.
Also, questions should go to the general moding forum. Not the turtorials section.
.
And even if I managed to, I don't know what to do with all those folders and files, where to move them or copy, and what does the text file referring to units using some Charle Magne unit animations has to do with all this? Again I am sorry, I usually have no trouble reading tutorials and guides 
.
.
