Rule 11. This is the snakes and ladders rule. If you 'fail' on a step, you must drop down the 'snake' until you arrive at the step represented by your remaining money - and start again from that step. E.g. you are trying to double £5-£10. You buy something for £2, hoping to sell it. You fail and have lost the £2, leaving you with £3. You are now back to step 10 (£2.56-£5.12).
Rule 11. Detail. Supposing you invest £80 and it goes horribly wrong. Instead of the expected £160, you end up with (say) £6. This can happen, don't worry about it. This means that you must consider yourself now to be back on step 11, trying to turn £5 into £10 (again). This sounds tough but you will realise the value of this when it comes to the larger numbers.
Put simply, you will learn that no matter how much you gain you can loose it all - and no matter how much you loose, you can gain it all back.