Speaking as an English teacher, you are at a pivotal stage of development as a writer, I applaud your awareness of your need to become better at spelling. While there are many paths to being a better speller, one of the fastest ways to become good at spelling is very simple. Each day, identify one word that you want to focus on.
Write that word in as many ways as you can think to do. For example, write it in small, medium, and large letters. Then get a different pen or pencil and write it backwards. Go to the sink, grab a bar of soap and write the word on the bathroom mirror with the soap. Go to a space where you can swing your arms wide and not knock anything or anyone down. Guess why? Yes, write the word in the air. Write the word using your right hand, then your left hand, then with your feet. Write the word using both hands together, then both feet together.
Write the word using only your imagination.
Write 10 sentences or questions using that word.
Speak the letters out loud.
Spell the word while you are deliberately laughing.
When you have completed all the above, go to the bathroom and clean the soap off the mirror.
Do one word a day. In a year, you will have 365 words solidly in your brain. And, have had fun in the process.