Download and install Temurin Java 21.
Make sure to activate 'Modify PATH variable' and 'Associate .jar', if they aren't by default.

Install the Minecraft Launcher from the Windows Store if you haven't yet. 😁

Download this 1.21.1 NeoForge installer.
Now download this NeoForge installer script to the same folder as the installer.jar.
(Full content of this script is available here.)
Run the NeoForge installer script by double-clicking on it in the folder.

Start the Minecraft Launcher. Go to Minecraft: Java Edition → Installations.

Click on the three dots to the right of the NeoForge profile → Click 'Edit':

Change the directory to something else than default (Like in this screenshot, a new folder called '1.21.1 NeoForge' inside the default Minecraft folder):

Now launch the profile for the first time:

Launching the profile for the first time has created all the folders needed. Press 'Windows'-key + R at the same time to open a Run-prompt and fill in: %appdata%\.minecraft

Then hit 'Enter'. This will open the default Minecraft installation folder:

Open the folder you created specifically for this NeoForge profile in the Minecraft Launcher:

Open the 'mods'-folder.
Download this mod pack Minecraft mods.zip.

Extract this .zip into the 'mods'-folder you have open and you're done!

Now you can restart the Minecraft Launcher and start the NeoForge profile to play the game!

See Connect.