If you want to download a custom shader.... thing, what do you open the file with?

You don't need to open it with anything. Just save it to Toribash/Data/Shader and open Toribash, and type in /lws shadername.inc and hit enter.

where exactly do I type in the shader name?

...

Hit enter, or "t". The regular fucking chat line >.>

where he typed shadername. Say the name of the shader is bannana, it would be /lws bannana.inc