I know there is already a thread out there for Optifine but here is a more up to date guide w/ screen shots. This guide is intended for players who are having issues with frames, freezes and more graphical issues. An alternative to this guide is just to use the CraftersLand Modded Minecraft Launcher which can be found HERE
Step 1: Go to Optifine website and click "Downloads" OPTIFINE WEBSITE
Step 2 Download Optifine.
Step 3 Find Optifine in your downloads folder then move to the mods folder of your modded Minecraft.
You have now successfully installed optifine into your modded Minecraft. To check and make sure its actually installed go to Options>Video Settings then it should display below as shown. Enjoy!