I use Magic Launcher. It's the recommended way, as it's made by the creator of OptiFine and also linked near the top of the first post in the OptiFine thread. With Magic Launcher, all you do is save the OptiFine download to a folder of your choosing, then from within the launcher just click "Setup" and select a profile (or create a new one). Click "Add" and navigate to the OpriFine download you saved earlier. Save the profile, and voilà! You can make as many different profiles with as many different mods or combinations of mods as you want. You can reorder the list by clicking the "Up" or "Down" buttons in the setup screen. OptiFine is designed to be installed last (at the bottom of the list) when used with in conjunction with other mods, if they're even compatible at all. Magic Launcher will tell you if there are any errors or compatibility issues.
http://optifined.net/adloadx.php?f=OptiFine_1.6.4_HD_D1.jar
It is an adfly link, so you will need to wait 5 seconds and click the SKIP ADD button in the top right corner.
Next, click on the download link then press save file.
A new tab should popup in the bottom of your screen. Open it and press INSTALL. This will install OptiFine into a new 1.6.4 folder.
Next, open up your minecraft launcher and go to New Profile. Scroll to "Versions" and select the 1.6.4 profile and click "Save Profile".
You should now be ready to play minecraft with OptiFine.
Village Mechanics: A not-so-brief guide - Update 2017! Now with 1.8 breeding mechanics! Long-overdue trading info, coming soon!
You think magic isn't real? Consider this: for every person, there is a sentence -- a series of words -- which has the power to destroy them.
It should still be the same.