Everything should be automatic, but it's not uncommon for transactions to fail to track. If that happens you need to file a manual claim, so it's worth keeping a note of all relevant info (precise time of order, order number, etc.).
I've been with TopCashBack for years without any problems. Last month I joined British Cycling through TCB and got £10 cashback, which means it's cost me just £14 for 12 months membership including the liability insurance.