Many thanks to Jadenn on esper/#minecraftforge for the right arg to use and smbarbour for pointing me to this tweet that set me up right: tweet that saved the day, and luacs1998 for getting the jar installer started (read: did most all of it XD)
READ ALL DIRECTIONS!
First things first: if you're wanting to directly modify the minecraft jar file you are in the wrong place. This tutorial takes advantage of the new local version system from Mojang, allowing us to customize our gaming experience without even touching the jar. If you wanna muck around in jar files revert to 1.5.2 or use modloader. This is not what Forge is about.
Second: Read the entire guide before starting, and read it as you go along.
Third: This method is no longer mandatory as you can now drop the OptiFine jar in the mods folder assuming you use a newer forge (tested on 18.104.22.1689), but it still remains useful if you use multiple versions of minecraft (1.4.7,15.2,1.6.2,...) and do not want to have to shuffle around your optifine file, or if you occasionally use mods that are not optifine compatible.
It doesn't get any easier than this. The entirety of this tutorial is both case and spelling dependant; if it does not work the first time *please* go back and check spelling and file placement. I'm almost certain this is where most issues lay. If this does not resolve the issue, please check the common errors spoiler before posting. If that does not solve your problem, please post the error log and tell which install method you used (manual or automatic) here so I can debug the problem.
And for Notch's sake, if you post an error log post the whole thing and use spoiler tags!
Also, when this works for you, please post a thankyou in the thread, one to keep this tutorial floating for the benefit of others, two because it makes me feel good XD.
[NOTE] For the purpose of this tutorial .minecraft will be used to show the location of your minecraft folder. Depending on your os it will be in one of 3 places by default. If you have a custom setup then you'll have to know it yourself.
Linux = ~/.minecraft
Windows = %appdata%\.minecraft
Mac = ~/Library/Application\ Support/minecraft
You will need to disable "Hide file extensions of known file types"; use this guide from microsoft to do that:
Show or hide file name extensions
Also, I suggest installation of notepad++ for text editing, as I'm writing these files on linux and it uses a different sort of line return than windows, making it all show as one line in windows. Notepad++ fixes this issue, and is all in all a superior text editor over notepad.
Play your game with both optifine and forge