Well, I certainly wouldn't know where to start, but I do think that if you take a look at http://www.bioweapons.com/
The webmaster has a Bio Booster Armor Guyver costume that looks incredible. You could probably apply the same sort of philosophy to a Spawn costume.
If you're the type that enjoys pain, then I'd suggest making a full-body cast and then sculpting the features you want. Then take a negative of that and make a latex copy, paint it, and you have an incredible looking costume. However, this is the toughest way to go, by far.
The easier way would probably be to get some foam, and attach it to a spandex suit so that you have a "Muscle" suit. Then run the spawn costume over it, so people who watch you get the impression that its actually muscles moving underneath the skin of the costume.
The sewing of the costume is pretty straight-forward, although it'd probably be easier to sew it as a two-piece.
Since few, if any people have actually attempted such a thing, its definitely up to you to research and figure out how you should acheive the look that you want. I'd suggest keeping a website or diary so that you can keep track of what you need to do to stay on schedule.
That's the most help I can give you, good luck!