You do not cast a spell to become a witch.
Becoming a witch is a matter of dedication to the craft and study, study, study. It is, in many ways, no different than becoming anything else one strives to be in life: a doctor, lawyer, singer, actor, dancer, etc. You must first learn the foundational information that allows you to progress in your chosen path.
Unlike the above mentioned mundane fields there is no one way to learn how to be a witch. You must strive toward that goal with all your being and understand that you will have set backs along the way. You will be ridiculed by those who are witches and by those who think witches are fantasy. The more you learn, experience, and grow the better you will become.
We have no experts in this field. Claims of such should be warning signs that someone is delusional and should be avoided. Go outside, experience nature, learn history, the sciences, learn about the natural plants and animals in your area, experience the seasons, read books on subjects that interest you (not just the fictional ones, try some non-fiction books for educational purposes). Use caution, not everything is what it claims or appears to be. Not everyone is who they say. Not all books are worthy of praise. Common sense is your friend. Check the bibliography of the books you read, check out the books the Author used, read those.
Above of all, think.