Installing Minechem
If you've already installed Minecraft and Forge, you should be able to simply drop the Minechem mod file into your "mods" folder.
To download Minechem, visit the CurseForge Minechem download page and download the latest release. Alternatively, you can download Minechem from the github releases page.
Minechem requires the CoFHLib to run. You can download it from the CoFHLib mod page on CurseForge.
If you've got the CoFHCore for another mod (such as Thermal Expansion), you don't need CoFHLib.
Once you've downloaded Minechem, simply place the file in your %APPDATA%/.minecraft/mods folder (along with the CoFHLib if necessary), and you're done!
