0light1levels15 Shaders is a core-shader. It runs in standard Minecraft without Iris, OptiFine or any other mod loader. You switch it on like a resource pack.
Enabling the pack
1. From the main menu or the pause menu, open Options → Resource Packs.
2. Click Open Pack Folder if the pack isn't in your resourcepacks folder yet. Drop the pack file in, then return to the game.
3. Find 0light1levels15 Shaders in the Available list. Click the arrow to move it to the Selected list.
4.
Checking that it works
1. Load a world and walk somewhere with mixed lighting, like a cave entrance or the edge of a torch-lit area.
2. Look at the blocks around you. You should see light numbers on them.
3. Stand next to a torch. Nearby blocks should show the brightest reading.
4. Walk into an unlit spot. Blocks there should show the darkest reading.
If you see markings in both places, the overlay is active. The pack uses very few computer resources, so you can leave it on while you build or explore. To turn it off, return to Resource Packs and move it back to Available.