Clint Smith, from Thunder Ranch training, has excellent videos on this on Youtube and guntalk.tv, among others.
But, in a nutshell,
When the slide locks back, bring the gun back to high chest level, muzzle still pointed at the target and gun slightly tilted to make the mag well visible.
Drop the spent mag, insert the fresh one.
Bring the free hand, that just inserted the new mag, up onto and over the slide, rearward of the ejection port, with the thumb pointed toward you.
With a sharp rearward snap of the slide, the slide chambers the first round.
With the gun still in the same position, close in high chest, regrip and then push the gun outward straight toward the target.
Experts do this in less than one second.
The videos make it much clearer, fur sure.