How can I write to God about my day?

I would suggest a simple hard bound journal or a nice diary. If you want it only for yourself, get one that locks. Find a nice pen that writes well so your script looks good and you’ll feel better about your efforts. You’ll develop your own style but a place to start is writing about your challenges and how you dealt with them. Write your goals and ask what you need to accomplish this. All this is intensely personal and best to not share this with others as it is an internal dialog between you and God.