Day 1.
Firstly, you need a Shyrack base.
Remove the Shyrack, then make a greensfuff square, this is the 'body' and push that on to the shyrack base.
Separatley roll little bits of greenstuff into 12 or so little snakes, these are the 'Tentacles'
Roll a greenstuff ball about the size of a marble.
Let these set.
Day2
Make a large greenstuff egg, this is the 'head', and press that onto the body, smooth it and round it off.
Day 3
make a 'beak' and add it to the front of the Body, make a socket for the 'marble eye' and add the eye to it.
Mottle the greenstuf to give it a fleshy look.
Add the 'snakes' to the sides of the body under the head.
And thats really it, its just about the paint job how much detail you add to the greenstuff as you are making it.
Use my pics for reference, pm me if you have any questions or even if you want one. You'd have to supply the Shyrack though
I'd use the Knobby spider base as the yammosk. Its all about keeping it cheap and to hand, so dont be chopping things that are expensive. 
ISP Speeder base 