First, run Minecraft 1.6.2. (you should have done this already)
Then, open up your .minecraft folder. You should know how to do this already if you've installed optifine in the past.
Open the new folder called "Versions". In this folder you should see another folder called "1.6.2". Make another copy of this folder and rename it too "1.6.2-optifine". (do not use spaces)
Open up this new folder and you should see a .jar file called "1.6.2", this is where you place the optifine files, the same way as in the previous versions. Also, rename this .jar file to to "1.6.2-optifine"
There should also be a "1.6.2.json" file, rename this to "1.6.2-optifine.json" and open the file using notepad.
On the first line you should see;
This needs to be renamed too.
It should now look like this^
Save that file and you're good to go. When you open the launcher create a new profile and use version "1.6.2-optifine"
I don't know if this is the only/quickest way to install mods, but it has worked for me.