Like previous posters always say, hold down the G key for the command map.
Aim for the choke points - i.e. where the enemy bunches together.
Don’t go aiming at random spots over the map, find a good spot or 2 and stick with them. It takes practice to learn how to move accurately between 2 targets.
Make your shots irregular so the enemy doesn’t learn your pattern.
Learn the spawn timing on the map (don’t forget to take into account whether the enemy command post is standing or not). Then fire 3-4 seconds early to allow for shell traversal.
I usually fire 3 or 4 mortar shells rapidly at that point - and can get the same number of enemy.
Don’t position yourself where field ops have to go out of their way to reload. You will sometimes get an intelligent field ops who will stay near and reload you (i like doing it!) but more often than not on public servers, it’s better to be encamped enroute to the battlefield so field ops can reload you as they pass, yet their charge refills by the time they arrive on the front line. Teamsay “i need ammo” and fire a shell before they run past - to give them a hint to reload you.
Thank your field ops - it’s greatly appreciated and makes it more likely you’ll be refilled.
Do onto others as you would want then to do onto you. If you ever fieldop - make sure to generously reload mortars. Later on in the campaign, chance are they’ll field op for you.
Likewise never fire your last shell - always keep one in reserve - as it’s sometimes difficult to recenter your aim without wasting 3-4 shells.
If you can block a path and force the enemy to take an alternate, longer route, let your teammates know. Then they got a fairly clear route to the objective. (shelling the centre courtyard near 2nd pump from the initial allied spawn in the Oasis map is a good wheeze)