Each cruise line is different; check their websites or brochures. Most lines let you choose to pay in advance ( a fixed amount) or to pay individually in cash at the end of the cruise - your choice.
Thomson cruises include tips in the cost of the holiday so no need to tip at all....although of course you may like to for excellent service.
Each cruise line tackles this tricky subject in a different way, which they will explain in their brochure. Check which method they choose BEFORE you book. It is no use moaning about the method AFTERWARDS! I personally object to a fixed about being taken from me when I book or worse, taken from my onboard account automatically before I leave the ship at the end of the cruise! I prefer to give my cabin steward and restaurant waiters their gratuity personally at the last minute before I leave the ship. You then give them how much that you think they deserve, not simply a fixed sum. Around £20 each for a weeks cruise would be OK. The exact amount obviously depends on a number of circumstances. The best way to deliver this is in a small "Thankyou" card purchased before departure, and just write their name on it. They will not know how much you have given them, so you will not have to worry about their reaction! Happy cruising .