Optifine is a wonderful thing!
It usually improves Minecraft's overall frame rate, gives you oodles of fine-grain display options, supports connected textures (mmm... smooth glass), and supports shaders if you have the right kind of GPU. I made one of these instructional things some time ago, but it's probably way long out of date. So, here's a refresher!
Step 1. Download and Install Forge
Download the latest installer jar from
files.minecraftforge.net. Run that thing! If it asks, you want to install the client, not the server.
Step 2. Create a "mods" folder
This is where you'll put Forge mods. Create a new folder called "mods" in ~/Application Support/minecraft/. Forge will automatically try to load anything in that folder that it thinks is a Minecraft mod.
Step 3. Download and Install Optifine
Download the latest version from
optifine.net/downloads. Put the downloaded jar file in the mods folder you just created.
Step 4. Start Minecraft using the "forge" profile
When you ran the Forge installer, it should have created a new profile for Minecraft. This tells the launcher to use Forge when starting the game.
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That's it!
It's gotten much easier than it was in years part. If all goes well, when you start Minecraft you'll see a different startup screen with progress bars showing Forge loading its things. You should also see something like this in the corner of the main menu screen to confirm that it's all there: