Install Minecraft Java Edition
Install a compatible mod loader, usually Fabric or Forge
Download and install Iris Shaders for Fabric or OptiFine for Forge/standalone shader support
Launch Minecraft once with the mod loader profile
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
Click Open Shader Pack Folder
Move the shader pack .zip file into the folder
Return to Minecraft
Select the shader pack
Click Done
Adjust shader settings if needed
Restart Minecraft if the shader does not apply