Level Design (help) transparent walls?

[Robsondo]

I've been trying to make objects like a drinking glass or glass walls but I can't seem to figure out how to change the transparency settings to make it partly 'see trough' like a real drinking glass etc.
It doesn't have to be breakable, I just need the object for decoration.

How to you make walls/sides transparent for like 50%? (Not invisible)

 

[Murphy]

Create a translucent image in a program such as Photoshop, Gimp or even something as simple as Paint.NET and import it following this guide:
http://liandri.darkbb.com/t75-importing-textures