Minecraft beta 1.8 was the first version of minecraft that I ever played, and I'm trying to play it again now, but I'm having some trouble installing optifine for beta 1.8.1 with the new launcher. I'm not sure this is correct, but I've been unable to find the minecraft.jar folder that I think optifine is supposed to go in? The Optifine I have for beta 1.8.1 isn't a .jar installer either, I think I have to put it in the game manually? I'm pretty lost and would appreciate some help.
Any old mods that refer to a "bin" folder with "minecraft.jar" now go in ".minecraft\versions\version\version.jar", where "version" is the name of the version you are using. You also have to make a copy of the version (copy the folder containing a jar and json file) and rename it, as well as open the json file and remove a "downloads" section containing a URL for "client.jar" (failing to do either of these will cause the launcher to redownload a clean copy of the jar) as well as change the "id:" "version" line to match the name you gave to the files, in addition to the usual installation instructions for "jar" mods.
For example, these are the installation instructions for my own mod, which is for 1.6.4 but is manually installed, along with Optifine (newer versions of Optifine can still be installed manually as the installer simply does all the above automatically, which is why modded profiles have their own version folder):
To install, create a new profile named TMCWv4 with 1.6.4 as the version and click Play to download it (or copy an existing 1.6.4 installation), then go to the versions folder in .minecraft (%appdata% on Windows) and rename the folder and jar to TMCWv4. Replace the json file with the one found in the root of this archive; it has already been modified so the launcher will not try to redownload the jar. Install the mod files directly into the jar using a zip utility (do not try to rename it to zip and use Windows because there is a class file in the vanilla jar with a reserved Windows filename which will break things), and delete META-INF (if you want to install Optifine install it the same way; do not use its installer. There are no conflicting classes and the order should not matter but I installed TMCWv4 afterwards; the only known conflict is the complete lack of void fog in TMCW, which Optifine cannot restore because I changed a different class).
These are the lines you want to edit in the json (using my own example, the json has a universal format so any version will be like this, aside from not having a server jar, which is irrelevant for a modded version):
Before (you can easily find this section by searching for "client.jar", without quotes):
After (note that everything between "legacy" and "id" was removed and the version was changed from "1.6.4" to "TMCWv4"):
"assets": "legacy", "id": "TMCWv4", "libraries":
It is very important to make sure you edit the file correctly (e.g. note the placement of commas and brackets) as it is by far the biggest source of errors when manually installing mods (I include my own json for this reason). If you are successful you'll see some warnings in the game output but the launcher will launch the game:
(this is an old screenshot but the latest launcher works the same way)