Step 1: Acknowledge that your inner child exists. It's ingrained in you, holding onto your childhood memories, coupled with the emotions and feelings of that time period. This goes along with the widely held belief that our bodies "keep score" of our trauma. Trauma can be many things, and how we tally trauma differs from person to person.
Step 2: Practice positive affirmations. They feel awkward at first, but eventually, becomes second nature. Start your morning off in the mirror appreciating yourself- you are important, you are smart, and you are kind.
Step 3: Picture a happy memory from childhood, if you can. Remind yourself that it was not all bad, and that positive memories exist.
Step 4: Get a really good therapist, if you can. Not everyone has the same access to quality mental health care, but if you do, take advantage of it! You can only do so much alone.
Although I've given you some steps into healing your inner child, just know that it's a lifetime process. Try to forgive yourself, and maybe even those that didn't give you what you deserved as a child.
Sources: https://www.apa.org/topics/trauma#:~:text=Trauma%20is%20an%20emotional%20response,symptoms%20like%20headaches%20or%20nausea.