Most of it is actually fairly easy to figure out - look at the belly of beast, the holes that the legs fit into are numbered, as are the corresponding legs. Check the legs near the body end.
The goblin rides in a howda/ sedan chair that a little experimentation will give you an idea how to assemble. The the sides have slots where the seat fits in, look for tabs and corresponding holes.
As for mounting it on a 50mm base, I ended up running a piece of wire up through the base and into the belly of the spider to give it a little more support, so the legs weren't bearing all of the weight. Disguised the wire with a couple pieces of cork so that the spider looks as if it is climbing over some rocks.
http://www.games-workshop.com/gws/catalog/...dId=prod1130264A close look at the GW site gives you a pretty good idea of what the finished item looks like and how stuff fits together.