Rule 3. During the first fifteen steps you must use the actual coins and notes gained in the preceding step. Let me explain. Supposing you are on step 9 and have just doubled £1.28 into £2.56 (say two £1 coins, a 50p, a 5p and a 1p). You must use these actual coins as your stake money for step 10, and not different notes/coins, or a check for £2.56. After step 15, it gets difficult to keep it up as the larger amounts of money will be in the bank and sales/trades will have to be done by check, etc.
After step 15 I urge you to set up a separate bank account just for this doubling operation, but this is not a rule, just a suggestion.
Rule 3. Detail. Supposing you have just made £40 (in step 12). This rule means you can't put it in the bank (or spend it) and then use another different £40 (cash or check) as your later stake money for step 13. This breaks the chain. In the initial tentative first fifteen steps this has a great effect. You are training-your-brain here.
By step 15, you'll believe in the system enough and it becomes less important. It is also tricky to deal in cash over a few thousand pounds.