Modloader

Download NeoForge from the following link. Use Minecraft Version 1.21.1. Use the most recent (topmost) version of NeoForge. (Usually the default).

NeoForge requires Java to be installed on your computer. If you don't have Java, you'll need to get it before installing NeoForge.
The "Windows Online" installer is sufficient, as long as you have an internet connection. (which you would already need in order to be looking at this anyways)