Installing a plimbing roof boot
	
	
		I have a small pool cabana, the roof of which has been covered completely with 30 year shingles.  I need to vent the bathroom up through the roof and consequently need to cut a hole in the roof (an oval shape), cut the shingles and attach the boot.  Can anyone give me step by step instructions how to do this (or give me a web site that can walk me through it)?  It is not clear if it is easier (or better) to cut the whole from the inside of outside etc. etc.
Thanks in advance for any help you can give me.