Say you wanted to install the Optifine mod for Minecraft and you've read my previous post on setting up Forge, Liteloader and VoxelMap for Minecraft 1.8 (or maybe you already have working mods). Installing the Optifine mod could be as simple as getting the latest 1.8 version of Optfine .jar file (specifically the 1.8 version since as of this writing the other 1.8 versions aren't supported yet by Forge or Liteloader) and sticking it into the mods directory.
Now, there is a good chance that this didn't work for you at all especially if you just read the setup post. I saw a number of posts on the 'net where people are asking why their Optifine doesn't work and people give answers who some say works and some says they don't. The main reason is that if you got the latest version of Forge and Liteloader there is a good chance that Optifine isn't compatible with it.
Go to the Optifind Download page and you'll see a link to the changelog link besides each version of Optifine. Read it and it will say which version of Forge it works with. If you have a version of Forge that is newer then that, Optifine won't work.
The solution is the install the latest version of Forge that works along with the latest version of Liteloader that works with that version of Forge. Then you can drop Optifine's jar file into the mods directory.