There is not much to say about GLSL Shaders Mod, really. It’s basically just a magic wand that adds awesome visual effects such as realistic lighting and shadows. You can call it either a Minecraft version of the real world. or a very realistic version of Minecraft. The world you stroll around alone every night has never been so glorious.
Do Note that this mod requires a graphics card powerful enough to render all these awesome effects. For easier use of the mod, installations of Minecraft Forge or FML are highly recommended.
If you cannot afford such PC power, there are similar mods that can also grant your wish about a more detailed Minecraft world, but no mods can provide you with such beautiful effects.
HOW TO INSTALL THE MOD:
*Without Minecraft Forge:
1. Start Minecraft launcher.
2. Close Minecraft launcher.
3. Run Shaders Mod installer.
4. In Windows Explorer, double-click ShadersMod.jar or something similar.
(Alternatively, you may run java from command line or batch file. This is the command to use “java -jar ShadersMod-v2.4.12mc1.8.jar”. You may have to use full path to java.exe if it is not in your PATH environment.)
5. Open Minecraft launcher. Select Profile ShadersMod and click Play.
6. Open Option screen. There should be Shaders button. Click it to open Shaders screen.
7. Profit (at long last.)
*With Minecraft Forge (strongly recommended)
1. Make sure you have installed Minecraft Forge.
2. Download GLSL Shaders Mod (download links are below).
3. Press start and type in %appdata%.
4. Find and open your .minecraft folder.
5. Locate to your mods folder (if this folder does not exist you can create one)
6. Copy Shaders Mod jar file to mods subfolder in your game folder.
7. Open the Minecraft launcher and select Forge profile.
Credit: karyonix | Source: Minecraft Forum