Install Minecraft Java Edition
Install the correct version of Java if needed
Download and install Fabric or Forge for your Minecraft version
Download and install Iris or OptiFine
Launch Minecraft once with the mod loader installed
Download a shader pack in .zip format
Do not unzip the shader pack
Open Minecraft
Go to Options
Go to Video Settings
Open Shaders or Shader Packs
Open the shader pack folder
Move the downloaded .zip shader pack into the folder
Return to Minecraft
Select the shader pack
Click Done
Load a world to verify the shaders are working