What shaders do
Shaders change how Minecraft renders: dynamic shadows, water reflections, light scattering through fog, and better clouds. They run on top of the game's existing renderer, so they need GPU power. A mid-range dedicated GPU handles most packs; heavier ones like SEUS PTGI need something faster. Integrated graphics can run lightweight packs but won't enjoy realistic ones.
Iris (recommended)
Iris is a shader mod for Fabric, Quilt, and NeoForge. It pairs with Sodium, so you get shaders and the performance improvements at once. Most modern shader packs are made for Iris.
Create or open a Fabric instance. Go to the Addons tab, click Add Addons, and search for Iris Shaders. Sodium is usually pulled in as a dependency. Then switch the addon type to Shaders, find a pack you like, and click Download.
In-game: Options → Video Settings → Shader Packs → pick your shader.
OptiFine
OptiFine is the older method and still common in Forge modpacks. GDLauncher doesn't list OptiFine as a mod loader, so the install is manual: create a Forge instance, download the OptiFine .jar from optifine.net for your Minecraft version, and drop it into the instance's mods folder. Shaders then go in Options → Video Settings → Shaders.
OptiFine conflicts with a lot of modern mods. If you're playing a modpack, check what the pack itself recommends before adding OptiFine on top.
Recommended shaders
For a solid default, try Complementary Reimagined, it has presets from Potato to Ultra so the same pack scales across hardware. BSL is the warm, vibrant option. Sildur's Enhanced Default keeps the vanilla look with minor improvements. SEUS PTGI offers path-traced lighting on capable NVIDIA GPUs (released through the developer's Patreon).
Performance tips
Most shader packs have quality presets, start with Medium or Low and step up if you have headroom. Render distance is the biggest single FPS lever; 8–12 chunks is reasonable with shaders. Volumetric lighting and shadow quality are usually the next things to drop.
If a shader pack won't load, check compatibility: some are Iris-only, some are OptiFine-only. Make sure your shader mod and the pack are both on a version compatible with your Minecraft version.