It is really not that complicated. You can (a) pre-pay when you book the cruise; (b) pay on board the ship. If you opt for (b), the tips will automatically be added to your S&S account at the start of the cruise. You can then either (1) leave them there, and settle the account at the end of your cruise with your credit card or with cash; (2) remove the tips completely, and pay them in case at the end of the cruise; (3) leave them on and adjust them up or down, and still pay them through you S&S account.
The options are all clearly spelled out in your docs and on the Carnival website.