Step 2: Mark and cut the coconut.
The next step is to draw a line with your marker around the circumfrence of the coconut, either dividing it in half or at whatever proportions you prefer your bowl to be. Remember that the half with the hole in it will be unusable as a bowl, and should probably be the more shallow of the two halves. To get an even line, I recommend standing a ruler on its end, with the coconut on a flat surface, and making a series of marks at the same height, then connecting these with a solid line.
When sawing along the guideline you've marked, I find the most effective way to get an even cut is not to saw all the way around the shell, but to pick one spot and work in deeper from there. My saw was fairly dull, so it worked much better to cut a narrow groove and then saw straight through from that point, using the guideline to keep the saw even. This step should take about ten or fifteen minutes, depending on how strong you are (and how sharp your saw is).