Hi Zed,
Relatively easy actually. If you follow the outline of your penis into your body, go past the scrotum, you'll discover a relatively hairless patch say about 1-2" past it towards the rectum and apply pressure on the hidden part of your penis there.
What really gets me off is when my wife hasn't cut her fingernails and just runs them along that part behind the scrotum. She has me literally squirming with delight.
Rhythmically applied pressure or even the us of a vibrator can be just as pleasurable. It sure floats my boat.
There is some more information on this website...
G-Spot
At the top you'll find a link to 'Male G-Spot', read the relatively short texts that are linked on the left.
This is a more indirect way of stimulating the prostate, primarily intended for people that, like myself, are a little hung up about having anything inserted into their anus. As I said in another post somewhere, the site looks like crap but has a lot of valuable information at hand.
Alternatively, you can always perform a search on your favourite search engine. I am sure there are a lot more pieces of information out there.
Hope that helps/clarifies the general idea.