Not sure what you can't understand: mesh files go into 'new_folder' folder, texture files go into the 'new_folder\textures' folder.There is something that I don't understand, you mention this: copy the .mesh files (in this case "Hoplites_early_ug0_lod0-3.mesh") into the "new_folder" folder and copy your new texture and the norm_texture (in this case "ionian_hoplite.texture") into the "textures" folder.
First you need to have the game unpacked. Then you need to get the proper name of the unit from the EDU (the one in the soldier line - example: soldier Dismounted_Feudal_Knights)I don't know where I could get the meshes and textures
Then you get a formatted version of the modeldb file because the original is a pain to read. Either here (vanilla) or here (campaigns)
There you look for your unit (example: Dismounted_Feudal_Knights)
And there you will find neatly listed where the meshes and textures can be found.