rcoreyus has it right as far as the angle but to me that's if you are facing his head only because you have to remember a man's penis is normally curved slightly towards his head.
To me part of this fun position has to do with the size of his penis and your size
( body wise ) The longer or shorter he is and the smaller or larger you are has to do with the length of stroke you both will have.
This will have a great deal of importance of how much you can move, so with that in mind you can be conscience of it, you see?
And again like Sweedart says, practice is where it's at so the more you do it the more it will come to together.
But now when you are facing his feet which in the long run can give you an O that is very pleasurable, you may have to be in a less of an angle. Either straight up or just slightly angled toward his feet.
It should bring a little more pleasure towards your clit and your G-spot
Slow and easy at first and then as you see fit.
It's an adventure so have fun with it and take your time.
One thing to remember sometimes a mans erection is not as effective when he's lying on his back as apposed to standing up, on his knees or in the missionary posistions,