Install Minecraft Java Edition
Download and install Fabric or Forge
Download Iris Shaders for Fabric or OptiFine for Forge
Download a shader pack
Open Minecraft launcher and start the modded profile once
Close Minecraft
Open the Minecraft shader/mod folder
Put the shader pack .zip file into the shaderpacks folder
Launch Minecraft
Go to Options
Go to Video Settings
Open Shaders or Shader Packs
Select the shader pack
Apply and return to the game