Install Minecraft Java Edition
Download and install Fabric or Forge
Download Iris Shaders for Fabric or OptiFine for Forge
Download a shader pack from a trusted source
Open the Minecraft launcher and run the modded profile
Go to Options
Open Video Settings
Open Shader Packs
Click Open Shader Pack Folder
Move the downloaded shader pack file into the folder
Return to Minecraft
Select the shader pack
Click Done
Launch or reload the world