first of all, you have to unpack the files from vannila. to do this, you need version 1.1 or better.
go to m2tw\tools\unpacker and run unpack all. this could take up to 30 min
now, go to m2tw\mods\Stainless_Steel_6\data\export_descr_unit.
do a search (CTRL+F) for english knights.
observe they use mount barded_horse and soldier English_Knights
go to m2tw\mods\SS\data\unit_models\battle_models.modeldb. open the file in notepad and search for mount_barded_horse. you should find this:
now, assuming you have not changed the location for the unpacker, do the folowing:
go to m2tw\data\unit_models\Mounts\Barded Horse\textures and copy
horse_barding_england.texture and horse_barding_normal.texture to m2tw\mods\ss6\data\unit_models\Mounts\Barded Horse\textures
go to m2tw\data\unit_sprites and copy england_mount_barded_horse_sprite.spr to m2tw\mods\ss6\data\unit_sprites
now do a search for english_knights.you shuld find this
so,folowing this, you go to m2tw\data\unit_models\_units\en_pplate_plate\textures and copy
m2tw_em_pplate_england.texture and mtw2_EM_Pplate_normal.texture to m2tw\mods\ss6\data\unit_models\_units\en_pplate_plate\textures
go to m2tw\data\unit_sprites and copy england_Dummy_EN_Plate_Knights_sprite.spr to m2tw\mods\ss6\data\unit_sprites
scroll a bit down and you'll find this
so, go to m2tw\unit_models\AttachmentSets and copy
Final Special Heads_england_diff.texture and Final Special Heads_england_norm.texture to m2tw\mods\ss6\data\unit_models\AttachmentSets
now scroll a bit down and you'll find the entries for english_knights_ug1:
repeat what you did above