This Guide is only supported for Fabric 1.15.2, Forge 1.8.9 and Forge 1.12.2.

Before installing 5zig, make sure that you have installed Fabric or Forge first, depending on which 5zig jar you downloaded.
Once you installed Fabric/Forge, head over to the mods folder (Usually directly located under .minecraft) and drag the 5zig jar file into it.

You can now execute the Fabric/Forge Profile in your Launcher and the mod should load like normal.

Do NOT double-click the jar file itself!

Please visit the official Forge Website or the official Fabric Website for more information about Forge or Fabric respectively.