Install a shader-compatible mod loader or client, such as OptiFine or Iris
Launch Minecraft with the installed shader support
Go to Options
Go to Video Settings
Open Shaders or Shader Packs
Open the shader pack folder if needed
Download a shader pack and place the file in the shaderpacks folder
Select the shader pack from the list
Click Done or Apply
Wait for the game to load the shaders
Adjust shader settings if needed