The ideal time to prepare is just before immersion. However, when necessary, it is permissible to prepare earlier in the day preceding immersion, so long as the body is inspected and wet and hair is combed again prior to immersion. This applies both on erev Shabbat and on regular weekdays.
1) Yes, you may prepare your nails and remove any skin you customarily remove well in advance of mikveh, so long as you check these preparations closer to immersion, in case anything has changed in the interim. (E.g., to make sure nails are still clean and a proper length for immersion and to check for any hangnails.)
2) Before sunset, you should comb your hair again and check over any preparations completed previously (like nails, as above) that you will not be able to complete once Shabbat begins. At the mikveh, immediately prior to immersion, you should inspect your body once more, and run your fingers through your hair and wet your hair and body in the mikveh water.
Please write back with any further questions. You may also find it helpful to read our site’s article on immersion on Shabbat.