If you do your preparations fully on Friday, on Motzai Shabbat you should just shower normally, comb through your hair, and check your body for any barriers. These final preparations need not be lengthy, you should just make sure to do what that is needed.
Once you complete these preparations you may immerse immediately. You should check in advance the opening hours of your mikveh for Motzai Shabbat.
You may do your preparations on Friday morning (after sunrise) instead of Friday afternoon.
Please see our posted question on preparing for a Motzai Shabbat tevilah for more details.
Please feel free to get back to us with any further questions.
B'Hatzlacha!