Windows may say “Windows protected your PC”. Click More info then Run anyway — that message shows up for every file that was not bought from Microsoft.
Step 2 · Run it
Close the Minecraft launcher if it is open.
Double click the file you just downloaded.
A black window opens and does everything by itself. When it says Done, close it.
Open the launcher, and bottom left pick Punchy! instead of your usual version.
Press play. Your world is right there, same as always.
What it does to your game
Nothing is deleted. It only adds files.
Your worlds, your builds and your stuff are not touched.
Your normal Minecraft stays in the menu. Pick it any time to play without the mod.
Punchy only changes how your hands and weapons look. It cannot break a world.
It saves a copy of your launcher settings before changing anything.