OUTDATED: This instructable no longer can apply as the clients I used aren't available anymore
Regardless, the information here might be useful to some people, salvage what you can from it.
Here is the issue:
You love the look of Shaders in Minecraft, but the GLSL Shaders mod is not by any means the most efficient in video rendering. You get a fantastic FPS boost with Optifine, and you tried to run the GLSL Shaders mod with it in Forge, but you run into an error - Texture mapping is glitched, and in some cases text doesn't show up as their proper characters, rather white boxes...
I found the solution to this after lots of research and figiting around with Minecraft.
Move along to find out how!
Step 1: Install Forge
This probably sounds obvious but go ahead and do it. Follow the link and download the 1.8 release.
Once you do so, check to see if you have a Forge profile in Minecraft.
If you can't find a Forge profile, click the New Profile in the bottom left corner and create a profile named Forge. Then, select Forge 1.8 from the Release tab. Hit save to create the profile.
Once you have the profile, run it once to make sure it works.
Step 2: Install Optifine
NOTE: There is a specific release you MUST use, if not you will run into errors or potential client crashes.
Go to Optifine and download the OptiFine_1.8.0_HD_U_D5. Make sure you don't download a different jar file, otherwise you will run into errors.
Careful here, please read.
The Optifine website and mod creators are funded by Ad Fly, which does mean you will have to sit through an ad. Keep in mind these ads are not "secure", so do not click the ad, go to the top right corner and wait until it says "Skip Ad".
Next you will be brought to a page with the download link. Click it and Optifine will download, but another ad will open in a new tab. Exit out of the ad.
Now that Optifine is downloaded, you need to run the Jar file. Run it and it should install a profile (just like Forge did).
If you don't find the Profile, create one the with the same process as you did with Forge. Then run it once and make sure it works.
Step 3: Install the Shaders
For this you will install a variant of the GLSL Shaders Mod (you will run into another Ad Fly page, click Skip Ad in the top right corner), install this mod and run it once (you might need to create a new Profile again). If it works, good job.
While you are here, install these Shaders (I like these best for performance/visual results, download the medium+ shaders if you want shadows) and put them into the shaderpacks folder. The shaderpacks folder is located at C:\Users\*****\AppData\Roaming\.minecraft\shaderpacks for Windows users.
The easiest way to get to \Appdata\Roaming is by typing %appdata% into your navigation bar.
Also, a large shader pack is available online containing several dozen shaders of different visual styles. Do a Google search to find a recent pack (as new shaders are constantly being developed).
Step 4: Do Some Tweaking
In order to get Optifine and Shaders to work in Forge, you need to launch OptiFine and tweak some settings.
4 things need to be disabled.
1. Anti-Aliasing OFF
2. Anistropic Filtering OFF
3. Fast Render OFF
4. Natural Textures OFF
Once those are all disabled, you may move on to the next step.
Step 5: Put Optifine and Shaders Into the Mod Folder
Navigate back to .minecraft (same place as the shaderpacks) and look for a folder titled mods, if it doesn't exist create it in .minecraft. In this folder, place the OptiFine_1.8.0_HD_U_D5.jar (the files you downloaded) inside. Then, run Forge to see if it works. Launch a single player or server world and see if Optifine works. Once that works, you then need to place the ShadersMod-v2.4.11mc1.8.jar into the Mods folder (make sure Minecraft is closed).
After this is done, you should be able to run both Optifine and Shaders together. If not go back over the steps and try troubleshooting it.