Place "textures" folder in "Data" directory in your fallout 4 install path
Make sure to add TEXTURES\ to the "sResourceDataDirsFinal=" line in the fallout4.ini
ex. sResourceDataDirsFinal=STRINGS\, TEXTURES\, MESHES\, SOUND\, INTERFACE\, MATERIALS\
If you want to change kits, simply rename whatever one you want to use to "prewarunderarmor_d" and replace the current file.
This mod replaces the "Undershirt and Jeans" item. If you don't have the item yet, you can use the command "player.additem FFC21 1" to add one to your inventory.