I don't have a tutorial on it, but I can tell you how I made a big tree before. Cut a piece of foamcore with irregular edges to be the bottom of the tree. Make a hole in the middle of the foamcore for a dowel to fit through, then take a handful of dowels and tape them together, with one going through the foamcore. Use floral tape to tape them together. Then add wires where you want the branches to be, wrapped in floral tape. When you've got the structure the way you want it, start covering it with fondant or gumpaste. It's fine if it's bumpy or has lines in it...adds to the tree look. When it's all covered, take a butter knife or a veining tool and add lines on the tree. Let the tree dry, then make leaves and attach them here and there.