How do I apologize to my bestfriend?

I am starting highschool after a hard time in 8th grade. I deleted all my socials and stopped going to school. That included losing contact with my bestfriend. We are so close and I found out she's going to my highschool, so I want to apologize to her for "Ghosting??" Her. I am starting school on September 10, I need all the help I can get.

Something I might want to put in is that she still loves me as her friend. When I was in school to have a meeting with my Vice Principle, we locked eyes through the door's window and she pointed at the door and yelled my name. That was our last connection. She didn't come in the office where I was, but she saw me and acknowledged me.

I am aware what I did was so immature but I was going through a rough time. I regret every second of it. I can't go back to the past but I can apologize. Any recommendations on how? I am thinking of texting her on Instagram. (I don't have her phone number because she doesn't have a phone) so I have to text.

But her messages are off unless she follows you back, she isn't on Instagram very much so I'm worried she'll respond after school starts. Any tips on how to start the conversation, apologize, etc?