Taking a long hot or warm shower is sufficient for chafifah as long as you take care to do your preparations thoroughly and not rush through them. Just be sure to take as long as you need in the shower to wash your hair and to ensure that your skin is clean, any scabs are softened, and warm water has cleaned all areas of the body.
You do not need to sit in the bath at the mikvah if your chafifah at home was thorough. You should, however, briefly rinse off in the shower at the mikvah, comb your hair through, and check your body for chatzitzot before immersing.
Please feel free to get back to us with any further questions.