If a woman has a total hysterectomy and the uterus is completely removed, she will never become niddah again. If only part of the uterus is removed (a subtotal hysterectomy), then she should discuss her operative report with a rabbi to see if she can become niddah again or not.
A woman who becomes niddah as a result of a hysterectomy, or who was already niddah before the surgery, will need to count seven clean days and immerse in the mikveh. The correct procedure is as follows:
In the case of a total hysterectomy, there is no possibility of uterine bleeding and thus she does not need to perform a hefsek taharah or any bedikot. She merely counts seven days and immerses.
In the case of a subtotal hysterectomy, a specific halachic question should be asked. If it is still possible for her to experience uterine bleeding, she will need to perform a hefsek taharah and at least a minimal number of bedikot during the clean days. This may not be possible for several weeks. Although the vaginal bleeding that follows a hysterectomy can be considered dam makkah and does not render her niddah, it will impede the bedikot of the seven clean days.
In light of the difficulty in completing the seven clean days, and the fact that almost all her bleeding is likely to not be from the uterus, she may modify the taharah process as follows to prepare for the first post-operative mikveh immersion: She performs the hefsek taharah examination, omitting the moch dachuk. She then counts seven clean days, but she does not wear white underwear, and she does only two bedikot: one on day 1 and one on day 7. (It is essential to remember the bedikah on day 7. If it is omitted, she will need to restart the clean days.) She then immerses in the mikveh.
A woman who was tehorah before a hysterectomy should still count seven days after the procedure and immerse one final time in the mikveh. She does not perform a hefsek taharah or any bedikot, and does not recite a bracha on the immersion. If this presents particular difficulty, a specific halachic question should be asked.