Here is my step-to-step project of building two Trebuchets for the use in my Bretonnian Army. One is going to be loaded, the other one has just fired. The results are available in the Gallery.

I don't like the original from GW much, being not even bigger than a cavalry model! The first inspiration came from this picture, I am trying to stay as close to that as possible. More background information about the functionality of a Trebuchet and many pictures can be found on Wikipedia.

Planning 

My first step was some planning, with a result on the right. The drawed "man" is a standard infantry miniature for size comparison, one square is 5mm on the paper. I designed the base structure to be about the same size of the original GW Trebuchet, while being overall higher and longer. I calculated the lengths of wood I needed, the next step is buying these lengths of Balsa or harder Wood.

Materials

Now I have all the basic materials, mainly consisting of wood: 3 x 1m of 4x4mm square wood, 1m of 6mm dia. round wood, 1m of 8x8mm Balsa wood + Greenstuff for details and some thread for the ropes. I already marked and numbered the sections for the basic scaffold, and added 1 extra mm to each measured length which will come off when sawing. I bought some balsa plates later, 3 mm thick for most of the planks. 2 x 10 mm would have been perfect, but I only found 3 x 100. So I had some more cutting to do before having the right sizes.
I was able to buy all materials for less than 10 Euro (+ Greenstuff), with lots of leftovers for other projects.

Assembly - Scaffold 

You can see how the basic parts interlock here ...we want a realistic structure, don't we? The sections where the beams crossed were sawed to the middle on both parts, and glued using wood glue (PVA). This also fills some smaller gaps. It helps alot if you mark the positions correctly before sawing.

The finished basic wood structure, with a Balsa base can be seen on the left. Because some supporting beams didn't fit well (I should have measured more carefully), I filled the gaps with small Balsa pieces and glue.

Assembly - Throwing Arm

The throwing arms and the basics for the counterweights are now ready. The round piece was rasped to make it thinner at the end. Then I connected the two parts for the arm with a nail in the round (hard) wood, spreading it a bit. After that, I removed the nail's head with some pliers and the end was put into the bigger Balsa part. Everything was sealed with lots of glue.
After knoting the rope to the wood, I wrapped the thread around the arm upon a layer of glue, to strengthen the connection and keep the thread in place. The knots got some extra glue, as I don't want them to open... ever!

You can see the basics for the counterweight on the right. The bottom is 3mm balsa, the ends 2 pieces of the 4mm square wood, and the top 8mm square balsa.

One of the finished throwing arms is below. For better transportation, it can be removed from the crossbar. I glued some heavy nuts and bolts into the empty space in the counterweight to make the arm stay balanced upright (like just having fired) when attached to the Trebuchet. The arm of the second Trebuchet will be ready to fire, it is glued on the crossbar lowered (see below)

Assembly - Metal fittings

I put some thick paper in certain places to represent metal fittings. Later, the nails on them are made of thin slices of a ~1.5 mm diameter Green Stuff bar, glued on the paper with PVA glue.

Assembly - Sling and handles

The sling was made of thick tin foil. The edges were folded to make them smooth, and I glued another foil on the back of the open sling (the one that has just fired). The other one was folded around a "stone" made of styro-foam. For holding the slings I put a piece of a toothpick at each end into the folds. The rope was then glued to these ends and I additionally fixed it with thin wire.

The loose sling of the ready Trebuchet was put onto the hook at the end of the beam, which I made of the end of a toothpick.

You can see the handles at the left. All parts are toothpicks glued with strong glue. The crosses were finished first, then I glued one at each end of another toothpick. The ends and intersections, as the mounting were made of Greenstuff. I also added a simplified holding or locking mechanism.

Finalisation

The last part was to add the nail heads on the fittings as mentioned above and to add some "Bretonnian flair". I added various parts from the plastic sprues, like a shield, a small lantern, some of the shoulder-shields of the knights (they fit well on all wood beams) and some gear of the peasants.

View the finished, painted Trebuchet in the Gallery!