I obtain the pattern from the original canopy. I cut one triangle and add the seam allowance. After that I copy the whole 8 pizza shape triangles onto the fabric. Remember to put the pattern cross grain onto the fabric. I Read this article from THREAD Magazine.
My problem was when I need to cut the pieces. My umbrella is huge, it is a Patio (Garden/ terrace) umbrella. I should cut the pieces on the large flat surface, which I don't have. I ended up cutting the pieces on my bedroom floor..... the cleanest floor in the house. It was difficult, especially when you aren't in the mood to do it.
After I miraculously managed to cut all pieces, then comes the fun part which is assemble them together.