OK...here's a quick run-through ( from memory..I am at work!! )

Step 1: The physical model ( CAS file )

All levels of detail used should be ;placed in the [RTW]/data/models_unit folder

Step 2: Textures ( DDS file )

This should be placed in the [RTW]/data/models_unit/textures folder. You may need to create this if it does not exist in your install.

Step 3: Text editing!

Currently, you need to update 5 text files to make this work, and you need to be VERY careful with naming. If you are taking stuff from another mod to use, follow the names they use in each text file EXACTLY or it probably won't work.

Text file 1) descr_model_battle

This defines the unit name and the model and texture used. Copy the entry for the unit you want ONLY. There will be an indicator on this bit of text which shows what textures are used by what faction if th eunit is used by more than one. Make sure the list includes the faction you want this unit to be in.

Text file 2) export_descr_unit

This defines the unit stats. Again, make sure the names in this that refer to the actual game model are identical to that in descr_model_battle. Also make sure the faction name matches the faction you want. Copy and paste in the section of text from the mod.

Text file 3) export_units ( found in DATA/text )

Add in the unit description here

Text file 4) export_descr_unit_enums

this gets a short description. Again, paste in the section you want to add

Text 5) export_descr_buildings

You don't HAVE to do this one, but if you don't your new unit will ONLY be available in a custom battle, and not in campaign. You need to add a line of text to the appropriate barracks type building and level to make the unit buildable

Get all those steps right, and you have a working new unit!