First, you select a dead body NPC from Bethesda's list of NPCs that have DEAD in the name. Drag it into the render window and put it in there, hovering above the ground. It will be standing up normally. Double click it, and give it a REF name, like MyDeadBodyREF.
To play around with its posture as a dead NPC, click the havoc button on the GECK. The body will fall and tumble a bit. Then, unclick the havoc button, and look at what you've got. That's your tool for making the body take a particular pose while dead, you do that repeatedly, perhaps letting it bump nearby objects, until you get the approximate SHAPE that you want. Not necessarily the place, but the shape.
Then, you move the body into the location you wanted. You could hit the havoc button once it's there, to see if it settles perfectly the way you wanted, if its helpful.
When the player loads the cell for the first time, havoc and collision will start affecting your newly-placed corpse and depending on what you're doing, it may screw up what you've made. If you see this happen, what you do is - - in the frame the player enters the cell, run this script command against your REF name:
MyDeadBodyREF.SetActorsAI 0
Script command is this one:
http://geck.gamesas.com/index.php/SetActorsAI
Then, when you go in game, you'll see your corpse perfectly positioned.
heheheh. I'm having fun with this!