You'll want to keep this in mind:
-Modpacks need a modpack launcher (Twitch, Technic, FTB, AT, MultiMC, etc.) to run, not the regular launcher.
-Versions are important, 1.12.2 & 1.15.2 are not compatible, as well as Modloaders (Forge, Fabric, Liteloader, etc.)
-If you want mods from different versions to run, separate them with folders, 1.15.2 and 1.12.2 mods will crash if they conflict and expect Forge 1.12.2 or 1.15.2 to be able to work as changes happened between them. Sure you get 1.9.4 & 1.10.2 mods to work but it's not common.
To install Forge you'll want the Installer from their official website:
http://files.minecraftforge.net/
1.Get the Installer, download it/besides the ad site and then go to your downloads. http://files.minecraftforge.net/
2.Double click the Installer, make it client and it will do the rest once you hit install.
3.Go to your .minecraft folder (%appdata% in the Windows search bar makes it easy to find besides the regular launcher if you go to the profile and then the game directory browse button).
4.You can put mods into the mods folder, but put a 1.12.2 or 1.15.2 to separate them (1.12.2 and below it will make mods work no problem, 1.15.2 mods don't load in extra folders you have for separating so you have to move mods in and out of the mods folder to load them, 1.12.2 this is no problem).
If you want to make sure Forge installed properly check your launcher once you have closed it and relaunched it, if a Forge profile isn't there create one, same with the mods folder, create one if one didn't when Forge was installing.
There is many install videos, you just have to find the right trustworthy one out there as many as quick to say 'install 1.13.2' and it's installing 1.12.2 when the 1.13.2 one wasn't out yet or otherwise.
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