如何记录流入会计系统的资金的交易？

I've been reading about accounting patterns described by Martin Fowler in his book "Analysis Patterns - Reusable Object Models". I understood the basic concepts: account, entry, transaction, etc; but I am not quite clear when it comes to the money flow when external funds flow into the system. To make sure that no money is created or destroyed, I use transactions (2 legged) so that each entry in the transaction is the opposite of the other. Say I am a contractor, when a customer pays me 00, how should I record this transaction? I've been reading about accounting patterns des