This project is not to be done all at once, it took me over three hours to make the entire thing. (However, if you have friends helping you, it will take much less time.) This project is much easier to do in Creative mode, but if you are willing to try and you have a lot of resources, you might be able to do it in survival.
Step 1: The Setup
The main material you will need for this build is diorite. I know in Survival mode, for me at least, diorite is the thing I throw into lava whenever I find it in my inventory, so you might want to use some cheats or just build it in Creative mode, like I did. (For those building in survival, you need about 1,800 blocks of diorite, or a little over 28 stacks.) You will also need andesite, though not nearly as much as the amount of diorite you require. (You need about 150 blocks of andesite, or 2 1/2 stacks.) For the eyes and mouth, you will need redstone blocks (or red wool,) and coal blocks. (You only need about 10 of each.) I suggest building this in a flat world, so you have the space.
Step 2: The Body of the Ghast
The body of the Ghast is the most time consuming part of the build, and the most boring. (In my humble opinion.) First, you need to start by making the frame of a 16 by 16 by 16 block cube out of diorite. Then, you have to fill in the sides of the cube, all six faces. Next, go two blocks in on each of the corners and make the first four 'tentacles', which are two by two columns extending 10 blocks from the bottom of the cube. Make the next five by moving three blocks to the side on each 'tentacle', and placing another 2 x 2 column. (Check the pictures for reference, they are much clearer then my explanation.)
Step 3: The Face of the Ghast (Markings)
After you are done building the body of the Ghast, I suggest taking a break (because you will probably have a headache for staring at a screen for so long.) After you are done with your break, it is time to move onto the steps that turn your giant, (and somewhat ugly) cube into an easily recognizable ghast. First, the mouth. Refer to picture one. (I can't explain this part well, so comment if you have any questions or suggestions, I am so sorry I can't explain this better.) I added the redstone torches to the eyes because I thought it made them better. Refer to picture 2. The markings on the face are a little more complicated, and are sadly some not symmetrical. Again, I just can't really explain this, so just look at the pictures and comment if you have tips or suggestions for explaining this.
Step 4: The Top of the Ghast (Markings)
The top of the Ghast is the simplest part of the markings, and is thankfully symmetrical. Like I said in the last step, I cant explain this so just look at the picture.
Step 5: The Back of the Ghast (Markings)
From the top of the ghast, extend the four strips of andesite down onto the back. The picture will tell you how much.
Step 6: The Left Side of the Ghast (Markings)
The left (left when facing the front) side of the ghast should look like the first picture. For the markings, refer to the second picture.
Step 7: The Right Side of the Ghast (Markings)
When you are done with the left side, go to the remaining side (it should look like picture one) and follow the markings on the second picture.
Step 8: The Interior Decorating of the Ghast
While the interior decorating is completely up to you, I have a few suggestions for you. If you look at the first picture, you'll notice (I hope) that the inside of the ghast is quite dark. What I found works quite well is glowstone, and if you look at the picture you can see the glowstone is inset into the floor. Under each of the eight glowstone sections is a leg/ 'tentacle', so you can put glowstone there without it being seen from the outside. And for the entrance/exit for the ghast, I thought the mouth was a good exit. Pictures four to eight show you how to build a simple pressure activated piston door. The last picture is my 'interior decorating' of the ghast; I made it into a redstone fortress, complete with arrows shooting out of its forehead, and fireballs out of its eyes. Please do not copy my redstone fortress design, I just wanted to give an example of a way you could decorate.
Thank you for reading, and if you liked my instructable, please vote for it in the Minecraft Challenge and comment pictures of your custom ghasts.