: Download OptiFine HD for 1.8. Place the .jar file into your .minecraft/mods folder. This provides the necessary "Shaders" button in your video settings.
For Minecraft version 1.8.1, there is no official, stable release of the standalone Shaders Mod. To use shaders in this version, you should use the of the GLSL Shaders Mod or, ideally, use OptiFine , which eventually integrated shader support directly. Installation Guide for Minecraft 1.8.1 Shaders Mod 1.8.1
The mod itself only provides the "engine"; you need a "pack" for the actual visuals. How To Install Minecraft Shaders For 1.8 (Step by Step) : Download OptiFine HD for 1
: Run Minecraft 1.8.1 at least once from your launcher to ensure all base files are present. For Minecraft version 1
Since 1.8.1 was a transitional version, the most reliable way to get shaders working is through a Forge-based setup:
: Download and install the Minecraft Forge version compatible with 1.8.