Once you have the outline roughed in, you can go in with the grinder and create your final shape. Here, we're using a tabletop grinder, but you can also use the grinding stones on your rotary tool. They can be very helpful in getting in to the more detailed areas.
Don't worry about any heavy grind marks or mistakes. These are meant to look a little rough to accentuate their individuality. I actually prefer the batarangs with the circular saw marks on them to a highly polished batarang. They're one-time use items, after all.