It really depends in what areas and in response to what it's tanked (eg "I feel like I'm annoying and no one wants me around" is a different type of low self-esteem to "I feel incompetent and like I can never do any tasks right")
But a key is building up trust/faith in yourself. So that means baby steps done REALLY well. Like making a small commitment and keeping that small commitment like it's a blood oath. There's this idea of "re-parenting" (being the parent you needed when you were a kid). Kids need encouragement and non-judgemental compassion from their parents, but they also need a regular schedule and boundaries. What makes a parent trustworthy to a kid? You do those things, and you start to trust yourself, which is much the same as confidence and self-respect. Another example: kids want to be able to tell their parents that they fucked up without getting screamed at or belittled. If they can't trust their parent to respond kindly, they will stop being honest with them. Can you trust yourself to respond to your mistakes the way a kind, curious and compassionate parent would to a kid's mistakes? etc etc. Whatever applies to your situation
Another element is being really, really clear on your own values. That makes you immune to certain kinds of interrelational strikes to the self-esteem. Because you are ultra-clear with yourself, then when someone doesn't like you (which is inevitable, no one likes everyone), you have this core of "well, I know I acted in accordance with my values, so there's nothing I could have done differently. That's just how it goes." Whereas if your behaviour shifts between value sets, or you're changing to try and please people, then when someone doesn't like you, you're like "what should I have done different? who did i need to be? what's wrong with me?" etc etc and there's a lot of second-guessing and self-doubt
idk they're a couple of random tips. The other rec. I have is Aidan Wachter's Reclaiming Rite - I'll post that here later.