Here is a little tutorial on how I do one type of pine tree.
1. I start with two strips of fabric, one green for the tree and one white for the snow. The size depends on how big you want your tree to be. Just remember that almost half an inch will be lost in the seams. I like to make different sizes so that the bottom of the tree will be bigger than the top. I also like the green to be a bit wider than the white. On this one the white is too wide and the green almost gets lost in the final tree.
2. Cut right angle triangles from the strips. The number depends on how tall you want your tree to be. As you can see, there are no hard and fast rules. Just remember that trees are never perfect shapes so a bit wonky adds to the realism.
3. Arrange the triangles and decide if you have enough of them. Then you have to add the sky. I lay a triangle on the blue piece to get the angle. Then I lay the ruler over the triangle and cut the blue fabric at the same angle. I like to use reversible fabric because I often get this wrong and then I just flip the fabric over. I like to leave lots of extra fabric on the top and ends and trim it later. That's easier than ripping out or adding more when you start to assemble the tree.
4. Once you have the sky on all the pieces arrange them so that they are offset a bit. One of the top pieces has to have extra sky on the top so that you can offset them. If you forget to do that, just sew a piece on later. Most trees in the forest don't have perfect points on the top. You can also, at this point, make a trunk. Just sew a strip of brown to a blue rectangle and sew it to the bottom of the shorter side. Sew a plain blue piece to the longer side.
5. Trim the pieces so that they are square and sew the pieces for one side together. Then sew the pieces for the other side together. Finally, decide how much you want them offset, and sew the two sides together. Press well and then square off the whole thing.
Place the tree on your design wall beside your winter house and enjoy.