How to Prepare Your Minecraft Instance for Shaders
Create a separate profile, back up your worlds and record your configuration before changing renderers.
Back up the instance
Close Minecraft before copying your saves and configuration files. Store the copy outside the instance you plan to edit. If a change causes trouble, you can restore the original setup without guessing which files changed.
Record the requirements
Write down the Minecraft version, loader version, shader loader and required renderer. Match downloads to that exact combination. A similar version number does not guarantee compatibility, so read the file-specific requirements.
Make one change at a time
Launch your clean profile first, then add the renderer and shader loader according to their instructions. Test again before installing a shader pack. Keep a brief note of each successful launch so you can retrace your steps.