How to install shaders for Minecraft with PolyMC 7.1
For this, I will be getting Complementary Shaders - Reimagined with Fabric & Iris, and this shader pack is designed to work with Iris Shaders for Fabric. It is optimized for Iris, which is the recommended mod for using this shader pack due to better performance, compatibility, and features like asynchronous shader loading.
The steps in this guide apply to basically all other shaders. You can follow along with another shader pack but just research it first.
Get Fabric & Iris
Open PolyMC and select your Minecraft instance or create a new one, then click "Edit Instance".
Go to the "Version" tab and on the right-side panel, press "Install Fabric" and install.
Then, right below the "Version" tab, go to the "Mods" tab and on the right-side panel, press "Download mods" and select or search for "Iris Shaders", Install it. After that, also install "Sodium", as it is required by Iris.
Get the shader pack & add it
For example, get Complementary Reimagined from either Modrinth or CurseForge for your Minecraft version.
Then, in the same window from "Edit Instance", go to the "Shader packs" tab.
There, on the right-side panel, press "Add" and select the zip file.
Apply it
Open Minecraft, join a single-player world or a server, whatever.
Go to Options... --> Video Settings... and then go to the last tab,
Shader Packs... and apply ComplementaryReimagined_rX.X.X.zip. Then wait,
it might take a moment but it's nothing to worry about.