Hi Erin,
I would not advise using henna.
If you want to do it yourself, I would advise using a cap to put mor FINE brown strands through the blonde chunks. Then let your natural color grow out before you try anything else.
Maybe your friend's mother will help you.
Your hair isn't going to be exactly the way you want it until all the blonde grows out.
I wish I could offer you more help. The best suggestion is to go to a professional who has experience in hair color.