I think the starting point is just very basic self care. As a pp says, bath, quiet time, comfy pjs, that kind of thing. Honestly just babying yourself I suppose - slowing down and spending time just resting and being physically gentle to yourself.
Make that a habit over a few weeks, and then build in something very simple like writing in a journal or doing a simple guided meditation. Over time, the habits lead you to know yourself better. Which in turn causes you to develop compassion for yourself, which leads over time to better self esteem.
Habit formation and self care apps abound that can really help with this sort of thing. Ultimately, in order to change your thoughts you're going to need to change your habits to support those thoughts, so if you start there, you are already miles ahead tbh.
Self care and self love is about becoming, through habit and over time, the loving parent to the child that's still within you. As we all know, parenting is a long game and consists of years of tiny little interactions, paying attention, being patient and forgiving. Same goes for parenting yourself.