In the absence of a commander, use teamchat to relay to the other squadleaders/squads what objective you've defined for your squad (i.e., "Squad 3 defending Farm!", "Squad 2 attacking Field!") so there's no confusion of intent or overlapping of squads.

Do not underestimate the value of being able to spawn on a squadleader and having your squad continue to place unrelenting pressure on the offense or defense of a flag. Know when to engage and when to stay back, hidden from the enemy. Make sure your squad knows how to not give away your position when they spawn. On defense, when your flag goes gray, the squadleader's job is to stay alive at all costs so your squad can respawn to provide immediate reinforcement. Do not play the hero and try to find the one or two enemies on the flag. Unless you're Mantis, I guess, since he has a 100% success rate in everything he does, in and out of game.

Be wary of the enemy's most likely angles of attack (i.e. from the nearest enemy-controlled flag, or the most recent spotted enemy headed in your direction, or the usual attack routes based on the map in play.) Set up your defensive perimeter against these angles to concentrate firepower and overlap fields of fire. Instruct your squad to spot first, then engage. If the enemy is coming from an unanticipated angle of attack, this will give you the info you need to shift your defensive perimeter.

If you're not in a safe place for squadmates to spawn in, warn them. Let them know what kits would be most useful to spawn in as if there is a special need (i.e. if your flag is under attack by a tank, someone needs to come in as Spec Ops or AT... you would think this would be second-nature to most people, but you'd be surprised.) Make sure you are not in a fully-loaded vehicle so your squad can spawn in on you.

If you're new to squadleading, appoint a relatively experienced player as a fireteam leader or co-squadleader or whatever you want to call him. Delegate stuff to him (i.e. you're in charge of making sure we have the right kits in the squad for what we're up against, making sure the squad sticks together, etc) to allow you to focus on the squad's primary objective, squad positioning, putting down markers, talking to the other squads/commander, etc.

DON'T... be a sniper and plant random claymores around your position for your newly-spawned in squadmates to find.

DON'T... get freaked out by a squadmate spawning right in front of you and blow them away with a shotgun and then teabag them. Makes for an awkward moment on squad VOIP.