Kol hakavod to your mother for taking this on this special mitzvah!
Your mother should physically separate from her husband, wait five days, perform a hefsek taharah, count seven clean days, prepare, and immerse. If she had a total hysterectomy she does not have to perform any bedikot during the seven clean days. If, however, it was only a partial hysterectomy, she should perform one bedikah each on days 1 and 7. She should do the hefsek and any bedikot carefully and gently, and can moisten the cloth with water if necessary.
Since your mother was niddah up to this point, having never toveled before, she should recite the bracha upon immersion.
Please feel free to get back to us with any further questions.
B’Hatzlacha!