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As far as I know, the best way to act, if it happened and a robber is shot and laying on the ground.

Check the surrounding area, if everything is clear get any weapons out of reach if the thug is still alive.
Call 911 and say you shot an attacker, please send police and an ambulance may be required. Explain your location as detailed and possible. Describe how you look (blue shirt, black pants,..).
Hang up and wait until police shows up. Holster your gun and make sure it is concealed.
Most important, try to stay calm! Good luck!