I levelled a paladin as holy because I already had this warrior and I really just had him to heal. I can’t really recommend it unless you’re only doing dungeons because it’s pretty slow. But I’ll probably do it again anyway.
When you’re grinding in the overworld, you can be really safe and end each fight with full health and mana, but it takes a while and there’s a lot of autoswing.
Healing is basically just spam Flash of Light (extremely mana-efficient) until it looks like you’ll fall behind (or if someone takes big spike damage), then use Holy Light which is less mana-efficient but has higher throughput and can get big crits. Get into the rhythm of spamming flash, and anticipate who you’ll be putting your next couple of heals on.
Downranking really helps to save mana, especially once you get some +healing and Blessing of Light. The Illumination talent gives you mana refunds when your heals crit, so spell crit becomes the go-to regen stat, and Divine Favor allows you to force a crit heal on demand. Between holy talents and Concentration Aura, your heals are pretty much free from damage pushback.
If possible, I like to drop a Judgement of Wisdom or Light on the target (really good for passively healing pets or dps with fast attacks), or the one that keeps them from running away (Justice I think?). Be ready to use Blessing of Protection on squishy classes if they pull aggro (you can remove it by rebuffing them with Wisdom/Might/whatever). If it’s a certain wipe, use Divine Intervention on someone who can rez and be ready to remind them what’s going on because it sometimes takes people by surprise.
If you have other paladins in the group, you’ll have to coordinate who uses what blessings on whom. 5-min buffs can be a pain, but the best way to keep everyone buffed is to buff yourself first, then immediately buff each member of your party, then do this every 3-4 minutes. Your self-buff acts as a timer for the rest of the group’s buffs, so when your self-buff gets down to 1 minute or so, it’s time to rebuff the party again.
I think the toughest part about healing as a paladin is that you have to cast every heal separately and you can only heal one player at a time, so if people are spread out and everyone’s taking damage, it can get tricky. But when you really get into it and you’re feeling the rhythm of the heals, it feels pretty rewarding keeping everyone safe.