My followers keep getting so close to dying and I only know how to heal myself. I've seen others heal their followers but I can't figure out how to heal mine.
How do you heal your follower in battle?
Protected Aka "I'm invincible"
But you should not worry, most non-quest followers are immune to death by normal enemies. They are protected, if they get damaged enough to die, they crouch, and enemies stop attacking them. They can only die if hit by a random attack that was actually directed at somebody else, an area of effect spell, or if you the player hit them. They can and will die if poisoned.
You can also give followers healing potions. They will use them to heal themselves. A bit expensive solution perhaps, but it works. Of course, this might be buggy, and they might only use store bought potions. And do not give them fortify health potions. They will die if the combat is still going on when the potion duration runs out.
Changing the difficulty might help. Followers might be affected by the difficulty of the game and might take and do full damage. (not 100% sure of this one, but it used to be true for oblivion at least). But if the difficulty does work, then at higher difficulty they would both do more and receive less damage.
Skyrim helps those who help themselves
Look at the list of followers, and select one who has access to restoration magic. These can heal themselves. Talvas Fathryon from the dragonborn DLC is one of those followers. Or if you have 500 spare septims, get Marcurio in riften. But as followers are a bit suicidal at times, they are not likely to heal themselves when you want them to. Such is life, let them go to Sovngarde if they are eager to go.
They will not heal you. They hate you that way. You should have let them have that sweetroll.
Your droids, they are not welcome
You can also buff your followers (undead and automatons included) with the Illusion school of magic and the Master of the Mind perk.
Call to Arms grants significant combat bonuses to all nearby allies and enemies. If you cast it during the civil war quests before your allies engage the enemy they will easily be able to defeat the opposing side without much assistance from you.
It gives allies 100 in all combat skills, and increased health and stamina for ten minutes.