Natural light is by far the easiest way to get good results with macro photography. So going outside (preferably on an overcast day, to avoid bright reflections and dark shadows) is actually your best option. Even simply taking the photos near to a window might be good enough.
Otherwise, you need to diffuse the flash by placing a piece of white cloth (like a piece of handkerchief) over it. (It doesn't have to be cloth. Even an ordinary piece of copy paper, taped over the flash, can often do quite a good job as a diffuser).
Chris