Current ModSystems are Tweaks themselves, but can also load other Tweak-based mods as part of their startup process. What this means is that once you have one ModSystem installed, installing other tweaks becomes incredibly simple.
To install any other Tweak-based mod, simply drop the Tweak jar into your versioned mods folder, this is simply a subfolder inside the regular “mods” folder named for the version of minecraft, eg. for Minecraft 1.8 this would be .minecraft/mods/1.8/
Optifine doesn't require any additional steps, simply download it and drop the jar into the versioned mods folder.
ShaderModCore is a little more complex, but not much. How you install it depends on whether you are also using Minecraft Forge or not. Click one of the options below for details: