I made this last year but never finished in time for Halloween. So the project isn't perfect. But it should at least give you some ideas for your own project. Keep in mind that this was my first time trying any arts and crafts at this level.
Step 1: Get Reference Pics
Get some reference pictures, this is very important, I doubt you can make this from memory.
Step 2: Get a Rough Idea.
This part isn't necessary but if do it will help you visualize how the end result should be keeping you from making any unnecessary mistakes.
Step 3: Paper Mache
You want to get a beach ball, however big you want the head to be and cover it in paper mache. If you don't know what paper mache is it's just newspaper strips dipped in a flour and water mixture.
Step 4: Cutting the Parts
Now what I did was wrong. I cut the sphere into fourths and then cut out circles for my head and the sides of wheatley. You should cut out the holes first then cut the sphere into fourths. I used a marker on a string to cut the circles but you can use a compass. You should cut out one big circle and from that circle cut a little one out which you will cut in two to make the eyelids. After that cut circles in the side of the sphere, THEN cut the sphere in fourths. DO NOT do what I did and cut the sphere in fourths then try to cut half circles on each part. They won't fit well. IT IS ABSOLUTELY NECESSARY TO CUT THE SPHERE INTO FOURTHS OR ELSE IT WILL BE IMPOSSIBLE TO COVER IT IN FABRIC!
Step 5: Felt
Now cover each piece in felt. Covering the round parts is going to be hard. So you're probably going to need to cover it in two parts.
HINT: If you push the two pieces of felt together when gluing them they will blend into each other. It is also a good idea to spray paint the pieces white before hand because sometimes the newspaper can be seen through the felt.
Step 6: Adding Details
Now you can add some details like below.
No connect the four pieces with whatever you want. I uses some steel wire and hot glued the two ends to the fabric. Make sure it can't move. The paper mache is very flexible so if you can use the wire as a frame of some sort.
Step 8: Finishing Up
Know you can print out an eye for whichever personality core you want. and just put some Velcro on it so you can change it out. You will also notice that on my costume there is no bars on the top and bottom. But I'm pretty sure you can make some of your own. Also the red section hilighted in the picture is missing.
Step 9: Few More Things
This is just some additional stuff that i forgot to put in the rest of the instructions or had no place. When I made this I used paper mache. It would be better to use an acrylic sphere. Also to print out an eye on a sheet of plastic(picture below). I tried to do this but the color had a hard time printing for some reason. Maybe you will have better luck. That's about. Feel free to leave a comment or something and I will answer it.