Shalom Rabbi, If the day to go on the mikveh falls on Sunday night. after fasting
Can you make preparations before sunset?
She should not wash herself on Tisha Bav itself, rather on erev Tisha B’av. Then on Motzei Tisha B’av she should comb herself again, redo the rest it lightly. The same way she does it when she goes to the mikva on motzei Shabbos.
O:CH 554-8, M:B 554-18.