outside of having time on the point... usually you never really have the time to line up a slapper as you would during practice...
though I agree with the traditional setup... i.e. puck 3 inchess off the front toe area, the appropiate weight transition and typical contact...
if you are taking a shot on the rush, or a one timer, without much time, I'd suggest you setup as best as you can, where you know you have a proper hockey positon and can transition your weight to your front stick
if coming in, ya... at an angle will give you better options
getting good wood on the puck is the most important...
1. lower your bottom hand to the point on your stick where you maximize your accuracy and your power... this is different on different sticks, lengths and for different players
2. hit the puck with the heel of the blade just before the puck, the slapshot is more of a shaft loading shot, and you transition the most power through the heel...
It feels as if you are hitting the shot with the puck right on the shaft, right on the palm of your lower hand
the release is where it comes off your stick, through the blade... where you point where it to go (unless you leave it open blade to get the puck up fast)
3. biggest one for me is making sure you are cupping the blade, by bending your wrists and keeping the blade closed
usually if my shot fails me, its this... even if I make partial contact with the puck, it will still fly better than if I attempt to hit a puck with a partially open blade
its called a clapper, of think of clapping the puck with a closed blade...
there are people who dont' have to do this, as they have good enough timing to close the blade at impact, but for consistancy sakes... keeping the blade closed makes me more consistant