save/create a png file in the root directory of the model
name the file whatever you want your texture to be named
this is your texture file, you may need to find an image to use off google images or create it
in a 3d modelling software on the model assign the meshes you want to have that texture to a material under the same name as the png file (excluding the ".png") part
if you did everything correctly your model should have those texture parts