Don't be afraid of carving!
Step 1: Bake Cake and Prepare Fillings
If you are making this all in the one day is recommend making the milk chocolate ganache at this point so it's set enough for when you use it. Mine took a good 5 hours to be hard enough. I made the buttercream at the same time and left it at room temperature too.
Step 2: Stack and Slice
I started carving without any filling, just to get the basic shape. Don't be afraid to exaggerate the cuts as it will build back up with the ganache. I then deconstructed and filled with a small amount of buttercream (no jam, this could make it wobbly).
I had a photograph of a horse to work by and kept stacking sheet cake until the height looked right. Your cake might have risen more or less than mine so just go by eye. Sit in the fridge for about an hour or so, the buttercream will harden and make it easier to carve.
Step 3: Get Into It!
I kept it in the fridge and layered ganache, left to set again before doing another layer.
Step 4: Finishing Touches
I did have to put a thin stick through the face and neck after considering the speed bumps I had to drive over to deliver the cake but these were whipped out when it was presented.
Hope this helps!