Rule 2. You must use only the stake money during the current doubling attempt - you cannot supplement it with your own money, borrowed money, etc. You don't have to use all the stake money but you can't use more than the stake money. This is explained further in rule 4.
Rule 2. Detail. Supposing you are on step 13 (£20-£40). You see an amazing bargain which costs £30 and could be sold for £80. Sorry, but you can't add £10 of your own money to the £20 stake. You must either haggle them down to £20, or find another strategy. Think of 'outside' money as contamination to the special effect of the 'doubled' money. Don't let outside money taint your stake.