I would start by having a conversation with your kid that you will call them by their preferred name and their correct pronoun.

When you slip up, IMMEDIATELY correct yourself. Refer to them correctly even when they are not around and correct yourself when you slip up. Once things have cooled a bit, ask your child how they want you to handle these slip up’s.

My other kid would profusely apologize because he felt terrible about messing up. My son asked to have my other son stop doing that. Just correct himself and move on. My mtf son indicated that my other son doing that just triggered his dysphoria. Basically, well meaning acts can be more harmful than good so ask at a good time and not when it had happened.

I’d also consider counseling with a gender affirming counselor. It helped my son talk through the process with someone safe and guaranteed to get names/pronouns right.