Use a chopping board.
Use a bread knife - these have scalloped serrations.
Place the sandwich on the chopping board. Place the knife, blade edge down, on the sandwich. Place one hand over the blade back and the sandwich, thus holding the sandwich safely. Or place the blade on the sandwich and place your other hand on the sandwich next to the knife - curl your fingers (fingertips touching the bread) with your knuckles resting against the edge of the blade.
Draw the knife back and forth with a steady pressure.
Wash the knife with hot soapy water, rinse with hot water, dry and put away. Do not ever leave knifes in bowls of dishwater - this is very dangerous and can lead to permanent loss of function of fingers from accidental cuts.
All knives should be sharp. Counter intuitively a sharp knife is a safer knife. But obviously sharp blades must be kept out of reach of children.