You will need:
A piece of cardboard
Sorry I could not put everything in the picture.
Step 1: The Ears
Fold in-half a piece of black fabric (WARNING: make sure you fold it so the ugly side is facing out. Fold it pretty side to pretty side!) and then sew the long side down (WARNING: Make sure the fabric is the right size for your ear's). Now turn the ear inside out. Once you have 2 like this stuff them with your stuffing, stuff them very full so that they stick up when you sew them onto the hood of your sweater.
I didn't have a sweater I am just using a green shirt so I don't have a hood to sew them onto, sorry. The last step explains.
Step 2: The Arms
Cut the sleeve's of your sweater off under the seam that attaches the sleeve to the shoulders (otherwise it would unravel). Fold in-half a very large piece of black fabric, big enough so that your already cut off sleeve can sit on it with the edge of the sleeve can match up with the folded edge of the fabric with a inch between the folded edge of the fabric and the edge of the sleeve. Trace it with the chalk and remove the sleeve (WARNING: Remember, make sure the tracing is about an inch wider the your sleeve itself) and cut out the tracing. Put them pretty side to pretty side and sew the long edge down, turn the sleeve inside out and Use a needle and thread to sew the shoulder end to the shoulder of the costume, put the first one on-top of more and trace it to make another.
Step 3: The Make-up
Halloween make-up (Black,Green,White, and Red)
Look in the mirror to make white Halloween make-up circles around your eyes, then make the rest of your face green ad black lines up the middle of your face. Your face now is make-uped. Lost the black make-up so it is not in the picture sorry
Step 4: Don't Have a Hood?
A piece of cardboard
Green and black fabric
Needle and thread (or) sewing machine
Cut out two long 21x2 1/2 inch's (roughly, doesn't matter if it is exact) piece of black fabric and fold in in-half (WARNING: Make sure you fold it length wise and also make sure you fold it pretty side to pretty side, ugly side out). Sew down the two edge's leaving the end open (as shown in the picture) and turn it inside out, stuff them as full as you can with your stuffing. Set the ears you just made aside.
Make a tiny green pillow (however big it is is up to you) by doing exactly what you did to make the ears, instead of leaving the end open sew it closed after you stuff it full. The amount of stuffing is up to you, more stuffing make's it stiff and it will stick of your head, less stuffing will make it... you get the drill. I recommend less stuffing but (again) its up to you. Now all you do is sew the ears onto this.
How to keep it on your head
This is where the elastic come into it, simply stitch (WARNING: for this part you will have to use hand sewing) the elastic onto the bottom of your base (very well, may I note). Stitch it together the two loose ends (if you don't want to stitch them together then just tie them). The two (previously) loose ends will go under your chin and the base and ears will sit on-top of your head!
Remember to wear the black pant's (LOL)