What you are going to need to create a Zoidberg costume like my own is the following:
1 lab coat (you can get it embroided to say Dr. Zoidberg if you wish)
1 scrubs outfit, top and bottoms
1 Stethoscope (optional but we're going for a doctor look here)
1 Oversized Bart Simpson mask
2 lobster claw oven mitts
1 can spray paint (red)
White paint (or white out)
1 Red rubber glove
1 bald cap
Step 1: The Costume
(Scrubs, doctor jacket and stethoscope): www.allheart.com
Lobster Claw Oven Mitts: www.easternmarine.com/lobster-claw-oven-mitt
While these ship you can get to work on the mask which will take the most (but not really that much) work. The mask you can pick up online as well or at a costume store and/or Target.
Step 2: The Mask (not the Jim Carrey Movie, Lol)
The first thing you will notice about the mask will probably be the irregular shape on top due to the spikes but this is easily remedied by turning it inside out. This step does many things; it creates a flat top of which to work on which is good to make round later on and It pushes the ears in (zoidberg has no ears). When you do this step push the nose through so that even inside out the nose protrudes as you will need this latter on.
If you had trouble putting it (the mask) on due to the small size of the opening of the mask (like it was for me), there is a simple solution. Use the opening already provided as a guide and slice (or cut) upward until you reach near where the spikes would begin. You can use small strips of velcro on the ends to close it up after the head is on if you wish.
Step 3: The Mask II (Also Lacking Jim Carrey)
Cotton balls work well here as you use them in each of the fingers a a little in the hand area, enough to inflate it a bit but not so much it weighs it down too much and makes it look funny. Painting the glove now as opposed to pre-stuffed prevents cracking of the paint which would otherwise occur when the shape of the glove changes due to inflation by the cotton balls. Once this has dried apply super glue around the nose area and attach the glove over the nose via the extra latex glove you didn't cut away (kind of tuck it in and hold it there until you are sure it has begun to dry, thus making the tentacle mouth typical of Zoidberg. You can probably speed this up if you use a hair dryer to blow it dry faster.
*NOTE: You can use polymer paint if you are worried about health issues (available at craft stores) but you will need sealant as well to prevent cracking.
Step 4: Final Step: WOO WOO WOO WOO WOO WOO!!!
*I made some corrections on this as I stumbled my way through making this costume myself so you get a better product than I did and can take pride when you tell your friends, "I made this costume by myself" and watch them be amazed.