Hmm. That will make it hard to use a tripod or other support, though others may have some ideas. It would be good to mount the camera. That way you could take longer exposures if needed. Also, when doing close up photography, the depth of focus is extremely shallow, meaning it can be very challenging to get good crisp images hand held.
It is difficult to use on-camera flash because 1) it is so close it will burn out the picture and 2) it is probably pointed at the wrong area. If you must use on camera flash you may be able to bounce it using a piece of white plastic or cardboard. You may also need to put layers of tissue or handkerchief over the flash to reduce the light output. There are flash rings that surround the lens and put out soft light, but I doubt you want to invest in something like that for this purpose.
If you are using digital and have some time you may be able to experiment as well as take lots of images to make sure you get what you want. Experiment at home to get some ideas of what works.