How to Get OptiFine 1.9 for Minecraft
Now that Minecraft 1.9 is finally out, a lot of popular mods, maps, and texture packs are getting much-awaited updates, and while that's generally a good thing, sometimes it can get tricky. Some mods have a lot of moving parts, and that means that it takes time and testing to make sure that they come out working right, and OptiFine is one of those mods.
Currently, Optifine 1.9 is in Alpha, but don't worry, you can still get it. Here's how:
Go to the OptiFine downloads page and click the "Preview Versions" link. That will give you access to the most current builds of OptiFine -- as of the writing of this article, Alpha 2, Alpha 3, and Alpha 4 are available. But presumably the creators will continue to update the builds as they do more testing and fine tuning. If one of the builds doesn't work for you, try a different one! Remember to be patient, OptiFine 1.9 is still in Alpha.
After you've picked up the new OptiFine .jar, just install it the way you would any Minecraft mod. If you've got any mods or texture packs already installed for 1.9, I would definitely disable them. A lot of users are having issues with OptiFine and it's probably a good idea to remove as many variables as possible.
The creators of OptiFine have been doing a pretty good job of keeping the community informed, and if you want to stay on top of developments going forward, you should bookmark the OptiFine forum thread.
Right now OptiFine is listed as 99% complete, with the creators working on fine-tuning and polishing. That's good! If you can't get the alpha to work, just check the thread in a few days and see if there's a new build or if they've moved to beta.
That's all there is to it! Keep at it, be patient, and good luck!