Charecter Customization

Hello, I am fairly new to Unity and C#. I wish to know how to make a charecter customization so that the players can make their own charecter look like anything they want. I don't know how to do this, so can someone give me a C# script example? I don't wish to copy down a script, I just want to know the idea of how to do it. Thank you if you can help me, if you can't thanks anyway.

PS: If it's useful to know, I use blender for my models.