In battlefield Vietnam how do you use mortar?

From the Civil War to Vietnam War; with one exception, all mortars are the same...muzzle loaders/gravity fed. The one exception was the US Navy's Brown Water Navy "Riverine Forces" in South Vietnam; which operated the all aluminum 50 foot long SWIFT BOATS. Those boats, officially designated PCF-Patrol Craft Fast, were armed with a twin fifty topside above the cabin & one "over and under combination weapon", a fifty atop an 81mm mortar which was LANYARD fired (like a regular cannon).