I've included enough direction that you can follow along with what I've done, but this project is extremely flexible so please feel more than free to flex your creative muscles. If you try this project please post pictures.
Step 1: Get Creative
Some things to consider
Colour: picking the right colour is important, for instance blue-green tentacle on green lettuce turned out to be a poor choice
Cross-Section: the slices should look interesting on a plate
Overall Appearance: should look good from above
Appeal: do you want people to be too grossed out to try it?
Taste: I think trying to get jello to taste good is an uphill battle, so I planned on this being more of a centre piece than a menu item (the cranberry juice tasted pretty good, but no one ate it anyway)
Plating: how will it fit on the platter? is there a sauce to go with it? what's the garnish? I laid out some precut slices so people wouldn't feel bad about cutting it.
Concept: The idea I was going for was "what if alien was served at fancy dinner parties?".
Alien? you don't have to make an alien appendage. Preserved flesh takes on a certain translucence that would be interesting to recreate.
Step 2: Gather Matterials
I used flavourless gelatin so that I could control the flavour and colour of what I was making. I chose a white juice so that I could colour it however I liked. You could of course use Jello in whatever flavour you like. Adding condensed milk gives an interesting milky effect, but at the cost of transparency.
I didn't want to spend too much time on a mold, so I just used tin foil and formed it around some balled up newspaper. I think paper mache would work quite well.
You'll need some things to decorate your creation. The items must have a texture soft enough so as not to interfere with slicing, and have a pleasant flavour. I used grapes and canned peaches and pears.
Step 3: Mould
Once I was happy with the model I used it to form an aluminum foil mould, by simply pressing a large sheet of foil over it. Heavy gauge foil would have been ideal but I didn't have any, so I just used multiple layers (2-3 worked for me). I folded the edges over to add some stiffness and used additional balled up pieces for support. If your model is longer than your mould will be simple fold up the end (see third picture). The mould doesn't have to be all that strong, I could easily squish mine with my fingers but it was rigid under it's own weight.
Now wedge the mould into a large pan that will help keep everything stationary and make moving easy. Line the mold with plastic wrap to waterproof and ease extraction. Sandwiching a layer of something with an interesting texture under the plastic wrap might have interesting results. Without it I got a crinkly texture which was alright.
Step 4: Make the Gelatin
Whether making jello or Jell-O cut the liquid by half to make it 2x strength. This is important so that the tentacle holds together and can be sliced. I actually prefer the texture at this strength, so no worries about it being edible.
Make half of your juice into the first batch of jello, fill the mold about half way, and refrigerate. Hopefully you started out with more than enough juice and you've got some jello leftover: put it in the fridge, we're going to use it later. Pull on the plastic wrap to get rid of any large wrinkles and poke at it until the jello seems to fit nicely in the mold.
Step 5: Decorate
From the very beginning I decided that the tentacle would have bone running down the centre so that the slices would look like a cartoon steak. Making these multi-joined like a spine I thought would look interesting. To make the bones I tried to cut cylindrical pieces of canned pear, but this was extremely difficult and I should have gone with a more pear friendly shape. It's your alien after all, it can look however you like.
I wanted a contrasting colour and thought yellow peaches would look good. While I think the colour stood out well I wasn't ecstatic about the rectangular cross-section. I didn't have a good story for the yellow lines, maybe they were veins, or maybe nerves?
For suckers on the bottom of the tentacle I used red grapes cut into rounds, which I think turned out really well. I should have made the jello a little bit lighter though so that they'd be more visible.
When the first layer was completely set (1-2 hours) I arranged the pear and peach pieces on top of it. Remember the extra jello in the fridge? Once all your pieces are arranged melt some of it in the microwave (keep an eye on it this is very quick) and spoon it carefully over everything. This will hold everything in place as the fruit has a tendency to float out of place. Allow this coating to set in the fridge before covering with a second batch of jello, prepared as the first. When the second layer was set I arranged the grapes on it and covered them with a thin coating of jello just as with the peaches and pears.
Step 6: Serve
If I was doing it again I would have set out some nice serving utensils and made a gross syrupy sauce with tapioca balls floating in it served in a gravy train. I found some really interesting exotic fruits at the Asian market that would have looked good on the plate, but they were out of my price range.
Though quite simple I think this method has potential to make some pretty interesting creations and I'd love to see what you can come up with. If you try this the only thing I ask of you is to post pictures.