How can I install mods in Forge?

I've been wondering how to install mods in Forge for what feels like forever.

You can also use mods via Twitch, which I also did, but I have to log in when I want to play mods and when I want to play normally again, I have to log in again and again, so I ask how to use mods in Forge installed yourself or whether that is even possible.

90% of these are just a jar file, you just have to drag them into the mods folder and if there's anything else it should be in the description.

OK thanks