All mortars must be plugged.  If you are making and using your own plugs, see this page on how to do so correctly.

Plugged mortar tubes must be either placed in a secure rack or buried 2/3 their length in sand when being used to launch shells or mines.  Never bury a mortar in gravel.

Treat a mortar like a gun - never place your hands, head, or any part of your body over it at any time, especially when loading shells.

To load a shell, unwind the fuse, put the shell in the mouth of the tube, and use the fuse to lower it in until it reaches the bottom.  Never force a shell into the mortar - if necessary, use an L-shaped stick to push the shell to the bottom of the tube.

Before loading another shell into a mortar that's just been used, turn the mortar upside down and hit it against the ground to knock out any debris or burning particles.  Never allow debris to build up inside of a mortar.