Here is a step-by-step comprehensive guide on how you can download, install, and use the shaders within Minecraft 1.21.5 to ensure the best visual experience.

For Iris Shaders (for modern visuals and fabric users), go to the Iris Shaders page— https://irisshaders.net, download the Fabric version for 1.21.5, and then install it using the Fabric mod loader.