If we go by the most up to date version of Minecraft avilable. This should be a short a simple (2 minute or less guide) to installing Minecraft for Optfine 1.8+

I've written them down in text form for you below, but alternatively I made you a little 2 minute video that explains it much better than I could ever write it for you below:

Luckily this is quick and simple

1. Make sure to download the official version of optifine, find the correct version of Minecraft for you
   

 (In most cases you'll want the latest one available for your version of Minecraft however in rare circumstances you may need a specific version to be able to use it with other mods.)

Use the offical Optfine Website: http://optifine.net/downloads

1. Hit download, skip the adfly ad (top right hand corner)
   
2. Run Minecraft at least once on the version of Minecraft you wish to play with.
   
3. Click on the Optifine that you just downloaded and then hit the install button from the box that has just popped up.
   
4. Done! Hit play on Minecraft and go onto your world with Optifine goodness!
   

Let me know below if this has helped you and if not I'll try my best to help!


- Dan