Install Minecraft Java Edition
Download and install Fabric or Forge
Download and install Iris Shaders 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 shaders folder
Move the shader pack .zip file into the shaders folder
Return to Minecraft
Select the shader pack
Click Done
Load a world to use the shaders